Well, if you don't know what Kiss is, I don't really know why you're here. Most of the ways to get here are through other Kiss pages, although I suppose you could have wandered along one of the few that aren't.
At its most simple, KiSS (Kisekae Set System) is cyber paper dolls. You have a viewing program, and files are made for that program which are often (but not always) dolls, and you get to dress and undress them, moving items around. You can dress up Spiderman as Sailor Moon (well, you could, if anyone made a Spiderman set with Sailor Moon clothes). It started as a Japanese program, and many of the Kisses are of anime characters, although there is a larger number of non anime Kisses from non Japanese artists appearing every day. If you want to know more, check out the links page and other people will define it much better than I. Go to the Big Kiss Page if you're interested, you can get viewers for lots of platforms there, as well as a huge selection of dolls.