Diversity is how many different forms of a region of DNA exist within a group of critters. By way of example, in my building there's genes for red hair, black hair, blond hair, and brown hair. So for that region of DNA, there's quite a bit of diversity. But inside this room, there's only the genes for a one hair colour. Much less diversity. In some fish, you have life history strategies: some have genes for becoming huge, mature adults, while others have genes for spawning young. If all the members of a species who would spawn young die out before breeding, then there's less diversity - now there's only genes for becoming big mature adults left.
One way we test for diversity is to look at neutral areas of DNA - these are areas that don't do anything. Much of it is just repetitious garbage, copy errors, and viruses that snuck in and went dormant. Most of your DNA, as far as we can tell, is non-functional. Some of it is very patently useless.
Because it doesn't do anything, neutral areas of DNA (loci) are free to do pretty much whatever they want through mutation, and they're passed on in the population. We can look at this as a proxy for total diversity*, which is harder to look at.
So, I want to show you two groups of loci, one of the loci in Caribou, and one in Moose. The length of the bars shows how widely the locus of DNA can vary.The colours mean something, but nothing important for here. Which species more diverse? Why do you think that species is more diverse?*Kinda sorta. Lots of boring caveats go here.