Bumpmaps essentialy give more depth to a texture, making the texture look more 3d instead of a flat image. They are pretty cool if you know how to use them, a lot of players just put their original head texture on the bumpmap slot as well as their 128, 256 or 512 texture slot, which isnt the right way to do it. If you want to know how to correctly use it, im sure there are other threads that will tell you,