What does anti aliasing mean?

anti aliasing (AA) is the process of smoothing edges so they dont appear jagged. The number associated with it (2x, 4x, 8x, 16x, 32x) is the number of samples taken to smooth it. to make it appear smoother, pixels around an area are added together then an average color is found. basically, the higher the number, the smoother it will appear. however, AA is some of the most taxing work that a CPU/GPU will do and can quickly overwhelm it

the wikipedia article here shows very good examples of the different levels of anti aliasing

http://en.wikipedia.org/wiki/Anti-aliasing