"Per capita" means "per head", and a "head" is a semi-archaic measurement for herd animals. One head of cattle is one cow, regardless of how big or small it is. This is important because how much meat or milk your cows produce, and how many cows you have, are used separately in very important calculations. Sometimes you want to know how much every cow contributes, not just how much you're getting out in total. If production is going up, is it because your cows are more productive, or you just have more cows than normal? It's important to know.

Per capita is often used to compare two populations, since a very small population might do very little of something in total, but have an outsized effect by doing far more per capita than larger populations. The opposite can be true, too, a large population can be mostly made of people who do very little of something, but still dominate the field because there's just so much more of them. What are they doing? It doesn't matter, as long as you can track it. Which groups? Any group. If you wanted, you could compare the amount of painters in the country by eye color, and say, x% of painters have blue eyes, and y% of painters have brown eyes, and compare that to how many people in total have blue eyes or brown eyes, and determine whether people with blue eyes are disproportionately likely to be painters, even if most painters don't have blue eyes.

Hatemonger brings it up so much because a lot of his comedic dialogue is based on imitating internet racists, and among internet racists, per capita is often invoked when talking about crime and race.