From the IMDB:
As the aspect ratio of movies are rarely the same as the aspect ratio of a television screen, when showing movies on TV it is necessary to make sacrifices. "Pan & Scan" refers to the technique of chopping off strips from one or both sides of the picture when displaying. The areas chopped off are typically changed on a shot-by-shot basis, depending on scene composition. The main advantage of this technique is that it allows detail to be seen, the disadvantage is that shot composition is sometimes destroyed. Contrast with letterboxing.