Anti-aliasing is a way of reducing the jagged edges in an image. If you point a
video camera at a black diagonal line on a white background, you find that the image is not just black-and-white. Those
pixels that had part of the line in them are grey. These make it look a bit blurry, but smoother. Anti-aliasing applies the same pixel-mixing to all edges, noticeably improving the appearance of curves, diagonal lines, and
fonts.
Sub-pixel rendering like
Cleartype looks similar.