Aliasing artifacts appear when rasterizing primitives because primitives are approximated by a series of pixels that lie on an integer grid. Aliasing is especially bad when rendering diagonal lines (or edges of polygons that are on the diagonal) and wide points. Figure 19 shows how a wide point covers more of some pixel squares than others.
Antialiasing is a technique that reduces aliasing artifacts (or jaggies) by modifying the intensity of a primitive's fragment based on how much the primitive overlaps that pixel fragment. When performing antialiasing, OpenGL calculates a coverage value for each pixel fragment based on the how much the primitive overlaps that pixel.