In this chapter, the people choose to return to Egypt and are sentenced to die in the wilderness, wandering for forty years.
I already covered the background for this chapter pretty extensively in my commentary on Num 13, so read that if you want to see what I think about the promised land and the Abrahamic covenant.
This chapter directly continues with the prior chapter, with the people essentially freaking out again, this time over a prospective war against the intimidating Anakites. As before, their reaction is to complain, but this time they decide to actually return to Egypt. When verse 4 sasys "appoint a leader", they obviously mean that Moses's days are over. Later, they decide to kill Moses and Aaron, the central figures in their new religion, and at that point the LORD intervenes.
As I established before, the promised land is a significant part of the Abrahamic covenant, so in rejecting the one, they are also rejecting the other. Since the promised land (and the Abrahamic covenant at large) represent a reversal of the curse of Adam, turning away from the promised land is a symbolic acceptance of death. Egypt is symbolic of "the world", i.e. life under sin, in a couple of ways. First, the Hebrews are born under slavery just like all people are born under sin. Second, Pharaoh is a symbol of the devil who tricked Adam and Eve into sinning against the LORD. And perhaps most significantly, Egypt is positioned as the opposite of the promised land, because of the way that the Israelites are playing between one and the other. I.e. the LORD takes the Israelites from Egypt to bring them to the promised land, but now the people are turning away from the promised land to return to Egypt.
As a result of all this, it should not surprise my readers when the LORD declares that all the men who turned away from the promised land shall die. The promised land represents the garden of Eden, which contained the tree of life. Turning away from the LORD and eternal life (symbolically) must result in death.
That's the core of this chapter, and the rest is mostly just details. We see Caleb and Joshua express their faith in the LORD, because it is the LORD who brought them through all of their trials and will establish them in the promised land. The LORD says he will destroy the people, and Moses talks him out of it. This is basically a repeat of Ex 32 when the LORD threatened the same thing and Moses's response was basically the same. Unlike the prior instance, the LORD nevertheless declares he will kill all of the men over 20, which is what I talked about above. The spies, who spread the "bad report" are killed instantly for their unfaithfulness.
We see the symbolic number 40 again (like the 40 days of the great flood, Moses's 40 days on Mount Sinai, etc).
When the LORD commands the people to go to the wilderness by the Red Sea, that would take them south and away from Canaan.
Lastly, we are told that the people, in their grief, attempt to storm the promised land without the ark of the covenant (symbolizing the LORD's leadership) and Moses, the LORD's representative. It should come to us as no surprise when they are defeated, but it's still interesting that they would refuse to go up when commanded to go up, and they refused to depart when commanded to depart. It seems like the people just do the opposite of whatever the LORD says at any given time, like rebellious children.
And with that, this generation is condemned to die in the wilderness because of their unbelief. They refused to follow the LORD and would have killed Moses and turned back if they were not stopped.