Each game is a file size of 4,8,16,32,64,128,256....you get the idea. Most games do not use all of those blocks, so a trimmer will erase the unused blocks to shrink the rom to the game's actual size...thus saving you valuable space.
I use Rominator which does it automatically.
http://www.tpww.net/forums/showthread.php?t=85549