From The TV IV

Letterbox is the method of copying widescreen film to video formats that preserves the original aspect ratio of the picture. The letterbox format has the famous "black bars" on a standard fullscreen (4:3) television set. This is a result of the original widescreen image being shrunk down to fit your television screen, without losing any of the original image, unlike the pan and scan method. There is a common misunderstanding that when you see the "black bars" at the top and bottom of the television screen, you're not getting the whole picture. People believe that for some unknown reason, the top and bottom of the image was purposly blacked out to hide stuff from you. This is not true. Not only are you seeing everything, but you get the added bonus now of viewing the side material that was otherwise cut off when the theatrical movie was transferred to standard television.