It's an algorithm for displaying images in case of CRT. It is a portion of the subject "Computer Graphics", Google it to learn more about it. It performs clipping on an image to be drawn on the monitor making sure that it is limited by four sides when clipping is performed.
These might help shed some light: http://www.codeguru.com/cpp/misc/misc/graphics/article.php/c8965 http://www.cs.rit.edu/~icss571/clipTrans/PolyClipBack.html