In
Computer Graphics, one of the two possible approaches to solving the
problem of
aliasing (in other words, two different approaches to
antialiasing); the other being, kind of obviously,
postfiltering.
Aliasing causes the jaggies ... or the "staircase effect" when looking at images. Because of lateral inhibition, the eye picks up on the jaggies very quickly; so graphics people spend a lot of time looking at it.
A prefiltering approach is one where you solve the problem of aliasing by working out the colour of a pixel by using techniques that treat a pixel as an area rather than a point. For example, if you wanted to draw a straight black line on a white background, you would not just fill in the pixels along the line, as you would if you were using Bresenham's algorithm. Rather, you would imagine the line as a thin rectangle. Pixels that were partially covered by the rectangle would be partially shaded grey depending on how much of the pixel was covered by the thin rectangle.
Probably the most common use of prefiltering is in the use of antialiased fonts that you are probably reading this Web page in right now. Microsoft's highly touted (but grossly unoriginal) ClearText is an example of using prefiltering.