They are for fitting texture maps, mostly. They show how the UV mapping relates a 2D surface (your texture) to the 3D mesh of the model, you paint something on that part of the map Poser will put it on the part(s) of the model that matches. You can make your own in UV Mapper, but the ones you download are supposed to be labeled so you can figure out what is what, some actually are. You can also download them to get an idea about how good (or bad) the mesh is made, I often check out other model makers work that way.