In this chapter, the LORD establishes ceremonial laws governing bodily emissions.
As with the prior chapters, the focus of this chapter seems to be on the prevention of communicable diseases. There is a strong (and reasonable) implication that bodily discharges could be related to, and transmit, disease, so having such a discharge is enough to render a person unclean. Because having a bodily discharge does not positively identify a disease, the degree of isolation is much less than what we saw in chapter 13 for those "inflicted" with a skin disease. The required sacrifices are also smaller. Overall, it is a lower threshold to be declared unclean by bodily emission, but also an easier and cheaper process to attain ceremonial purity afterwards.
In some respects, I think this chapter provides some context for Lev 12 when I discussed the impurity following childbirth. At last we can see that this law applies equally to men and women, with the caveat that women are more likely to have bodily emissions because of the menstrual cycle.
Many parts of this chapter imply that those unclean, whether man or woman, remain in the camp. They would probably be expected to avoid unnecessary touching of others, because touching someone without washing renders them unclean for the day (v. 11). Clearly this is a less stringent treatment than what infectious persons are put through, which we learned was temporary exile from the camp.
Keil and Delitzsch point out we can connect v. 18 with the abstention from sex in Ex 19. Even though it is in context of disease prevention, the people are expected to be ceremonially pure when the LORD approaches them, such as during the founding of the covenant. This highlights the duality between the public health motivations and the religious implications. So many of these provisions are intended to contain any diseases by isolating people who might spread them, and yet that isolation extends to excluding such people from the tabernacle and religious ceremonies.
Overall, this chapter first establishes rules for bodily discharges in general, and then provides two exceptions for semen and menstrual emissions. Each of these cases has slightly altered rules, but in general this section is similar to what we have seen before. There are periods of uncleanness, various ritual washings, and at the end (for some cases) a minor sacrifice of two birds. The sacrifices are not always required, because e.g. v. 18 shows that a simple washing is sufficient, but in most cases there is a sacrifice. The sacrifice does not have an economic scaling factor because even the rich are allowed to pay the cheapest sacrifice. Probably the most frequent case that a sacrifice would be required is in response to a woman's menstrual cycle, and since that occurs relatively often, the sacrifice is small to compensate.
I'm not going to discuss this chapter in depth because while there are many details, it's all fairly consistent and not very important to the larger biblical story. All we really need to learn from this chapter is that ritual impurity can be transmitted from person to person, just like the diseases that this law is trying to contain, and in response those people who are ritually impure must be temporarily isolated from the larger society. They are also temporarily separated from the religious ceremonies of the Tabernacle because that is how they must respect the LORD's holiness.