In this chapter, several related events happen: the people continue complaining fiercely, the LORD starts punishing the people, Moses demands help, and the LORD agrees to gives Moses 70 elders who will share in his spirit.
Having departed from Sinai, the people resume their complaints. It's not clear to me if the complaining in v. 1-3 is the same issue as v. 4+. It's possible. In the first instance, we are told they were complaining "of adversity", while later the focus is clearly around the quality of the food.
In the first instance, we are told that the anger of the LORD "was kindled" and it burned on the outskirts of the camp. Fire is frequently used as a metaphor for anger, but in this case (almost humorously) the metaphor fire became a literal fire that burns some of the people to death as punishment for their complaining.
Their more serious grievance, beginning in verse 4, is a renewed desire to return to Egypt. This is nothing new, and even the topic of food is nothing new. For instance, see Ex 16:3. Note that the bible terms these people an "asp'suph", a mixed assemblage. This is the only use of this word in the entire OT. They have lustful desires, weeping out of their longing for the meat of Egypt. This is a stern rejection of the LORD's provision, that they would be weeping that they "only" have manna, which is provided for them supernaturally. It also shows an escalation of their complaining, that at first they complained there was no food, so the LORD gave them food. Then they complained they had no water, so the LORD gave them water. Now they are complaining that, while they have food, they do not have meat.
At this point, it leaves me wondering if they will ever stop complaining. One thing I have learned in my short life is that if a person is intent on complaining, he or she will always be able to find something to complain about, and I think that's the story here. To be sure, wandering through the Egyptian desert is a pretty harsh place, but they're not complaining about the desert, really. They are complaining about the manna, and by extension, God's treatment of them.
One other thing to keep in mind is the selective memory exhibited here; notice how swiftly the Israelites forget their oppression and how diligently they remember the few benefits of slavery. Literally one year ago the Israelites departed from 400 years of oppression and "bitter... hard labor" (Ex 1:14). The Israelites "sighed... cried out... because of their bondage...groaning" (Ex 2:23-24). And now, literally one year later, they are longing, lusting after their desire for the food that they ate in Egypt. It is stunning to think that they would willingly return to harsh slavery (and the mandatory murder of their sons, no less) in exchange for bread and meat, because they do not want to eat manna anymore.
There are a couple principles we can draw from this. The first is above, that an intent complainer will always complain. The second is that as people, we generally focus more on the bad things about the present. To wit, the Israelites are acutely aware of all of the negative things happening to them (no meat!), but they are completely ignoring or even actively complaining about the good things happening to them, like the supernatural food and water, the cloud of the LORD's presence, the tabernacle that goes amongst them, etc. They have an extremely unbalanced perspective on what is happening in their lives.
Lastly, this unbalanced perspective also applies to the past. About Egypt, they are acutely aware of the good things then (we ate bread and meat!), but they completely ignore the negatives like how the Egyptians would kill their sons, they were whipped or beaten, forced into slavery, etc. In particular they are focusing on the issue from which they are suffering now (namely, not having any meat) while ignoring the many other issues that have been alleviated by their redemption. They aren't getting beaten or whipped anymore, so they promptly forget that they ever were, and instead focus on their newest problem, disliking manna.
As an aside, this isn't the first time they have desired meat. Back in Ex 16, when they first complained about food and the LORD gave them both manna (for the rest of their journey) and meat (just on that occasion). They are now complaining about meat again, and this time the LORD is much less forgiving.
Moses, for his part, is definitely feeling the pressure because these complaints are largely directed at him (for instance, note how frequently Moses redirects the peoples' criticisms at the LORD, the Hebrews' divine sovereign: Ex 14:11-13, Ex 15:24, Ex 16:7-8, Ex 17:2-4). That's why Moses is complaining about not being able to "carry" the people, because all the people are basically expected Moses to take care of them and provide for them. In response, the LORD says he will take "the spirit" that Moses has and put it on 70 elders of the people, who we can presume would then be expected to help lead the nation.
This implies that there is some particular "spirit" upon Moses. Most bible translations will capitalize it, Spirit, to imply that it is the Holy Spirit. The NASB goes even further and personifies it, "the Spirit who is upon you". However, the Hebrew does not say that, it only says, "ruach", the typical word for spirit/breath/wind that we have seen many times before. Biblical Hebrew does not have capitalization at all, so all capitalized terms in an English bible are inserted by the translator of that bible, not in the underlying text. In this case, it's essentially a theological gloss where the translator is presuming the text refers to the Holy Spirit, and capitalizes accordingly.
So what is the spirit of Moses? Is it just some esprit de corps, some elan, that guides him in his zeal for the LORD, or is it a proper noun Spirit, like the spirit of God that rested upon Joseph while he was in Egypt? I think this question is answered later in this chapter, when the spirit is placed upon the 70 elders "and when the spirit rested upon them, they prophesied." This is the first use of the word prophesied (Hebrew, "naba"). This opens a whole new can of worms: what is biblical prophecy, how is it formulated in the OT, and how does that relate to the spirit of Moses? Each of these questions deserves its own answer.
I have already partially addressed the subject of biblical prophecy back in Gen 49, when discussing the prophecy of Jacob. Now Jacob did not "naba", which is our first clue that what we are seeing here is a different kind of prophecy than Jacob's. Jacob's prophecy is poetic and it's future-predictive, very similar to what many people think "prophecy" is. "naba" is something very different. To be brief, "naba" is "to speak or sing while under divine inspiration". It is used specifically referring to a form of ecstatic worship or speech, although the precise nature of what is said is never revealed to us. That's how we generally know that it's not meant to be predictive, because we are not told what the elders said. It is more important for us to know that they "naba"ed than that they were prophesying anything in particular. So the subject of "naba" is debatable, whatever they might be speaking or singing. The critical element is that it is done under divine guidance, although not always the LORD. Sometimes in the OT there are false prophets who "naba". So the term is used of the form of speaking and prophesying, and not necessarily a validation that it is from the LORD. In this case, however, it is very clear that the LORD is inspiring his elders to "naba", to prophesy, as proof that they have the spirit that was on Moses.
This is further evidence that the spirit upon Moses was the spirit of God, the Holy Spirit, because genuine naba implies divine inspiration. I see this episode as being another extension of the "royal priesthood, holy nation" statement from Ex 19:6 which I have talked about a lot before. Most recently, I said that the semi-priesthood of the Levites was a move towards establishing the whole nation as a priesthood. Now we can see there is a parallel (but separate) extension of the prophetic ministry from Moses to a group of the nation's leaders. Most strikingly, when Joshua complains to Moses about Eldad and Medad prophesying in the camp, Moses wishes "that all the LORD's people were prophets, that the LORD would put his spirit upon them!"
Therefore the priesthood is only one of several tracks that Moses (and the LORD) wish to expand over the entire nation of Israel. Unlike the priesthood, the prophetic ministry remains much less tightly defined through the OT, as it never has a clear mandate and job description like the closely regulated sacrifice and festival system. Still, if there's one thing made clear by this chapter it is that true prophecy comes from the spirit of the LORD, which is interesting because the spirit of the LORD was first imparted to man when the LORD breathed life into Adam. So it's as if all of mankind, by the very act of creation, has some measure of the inspirational power and insight of the LORD. What does it mean for the LORD to "put his spirit" upon people if the LORD's spirit is what empowers us to live in the first place?
I don't think we have a clear answer to that question, nor does this chapter seem intent on giving us one. What we can deduce from this text would suggest there are different measures or different spirits which come from the LORD and are given to specific people at specific times. Everyone on the earth has the measure of the spirit of God that gives life and vitality to us all, but there was some special spirit upon Joseph (which empowered him to interpret dreams) and another spirit upon Moses, which is here imparted to the elders of Israel and enables them to "naba", but only once. We should understand that the spirit remained on the elders after prophesying, because the purpose of the spirit is to qualify them for leadership, taking on part of Moses's burden as the spiritual head of the nation. It's the same spirit, but it seems to have multiple roles in this case, which should not be surprising if it is indeed the spirit of God.
In parallel to all this stuff happening with the elders, the LORD has a separate message for the revolting assemblage, each weeping at his tent: he will send them an excess of meat until they are sick of meat, which almost sounds sarcastic, but at the end of the chapter he actually does it. With a certain degree of irony, the people immediately go out and gather all the quail they can and begin eating it now that it has been provided to them by the LORD whom they scorn, thanklessly. This seems very presumptive to me, and the LORD responds by sending a plague amongst the people when they begin eating. For more irony, note that after this the people stop complaining about meat, but begin complaining about other issues (which we will see in the next chapter). So once again, the nation is a recipient of the LORD's provision, at which point they immediately forget what the LORD has done for them and move on to complain about whatever is next on their agenda. Note that ten homers is a tremendous amount of meat, hence why it must be spread all around the camp (probably to be dried for storage?).
Something that fascinates me about this chapter while writing my commentary is how the sections describing the rebellion of the people are interleaved with the sections describing the spirit of God resting upon the elders of the nation. First the LORD tells Moses he will put the spirit on the elders, then he tells Moses about sending quail to the nation for a whole month. Then Moses goes out and tells the people the word of the LORD, and then he gathers the elders. Then the elders come in and are filled with the spirit, and then the LORD sends the quail. Reading the chapter in this way gives us a sense of the progressive development of the spirit resting on these elders, which in itself should be a joyful occasion and a blessing to the nation, but at the same time there is a brewing rebellion against Moses and the LORD.