Anti-Ailiasing reduces the presence of "Jaggies" (Hard, jagged edges) on objects, smoothing them out. It makes particularly rounded objects look much better. some notes, Most graphics cards have the default set to "Application controlled" if yours is set to something else, and then you turn it on in an application, you're gonna run into serious problems with framerate, so check to make sure it's either Off in game, or "Application controlled" in the settings.
