A diversity statement is a one page document where you discuss your views on diversity. Some one half of schools (approximately) are requesting one. It's a difficult thing to write, because you don't know who's reading it. I'm just discussing my general philosophy, and I list some examples of where I've tried to help out. For example, I once wrote an instruction manual for international students and researchers, to help them follow administrative rules.
One of the hidden agendas of these kinds of essays is to see if the candidate is capable of writing.