Letterboxing is the process of adding black bars to the top and bottom of a movie or video after shrinking the whole image to fit a smaller screen, which otherwise could not accommodate the wide resolution of the film. This is done because most movies or films are shot in a widescreen format meant for the theaters, which is wider than the format used by standard 4:3 TV and 16:9 HDTV.
The presence of many aspect ratios and formats for visual media led to the creation of various methods to allow viewing of such media on different devices with different aspect ratios compared to the ones used by the film industry. Letterboxing is the most logical of these methods since it allows the display of the entire image, albeit in a slightly smaller scale, as opposed to simply cropping the sides and leaving the center square image if a widescreen film is shown on a 4:3 aspect ratio TV.
In order to make the wide image fit onto a smaller screen, it must be scaled down until both sides fit within the smaller aspect ratio. Since the image is a rectangle, this means that there are now areas on the top and bottom of the image that are blank. The most logical way to address this issue is to make these areas black so that they can be largely ignored.