What is letterboxing?
Letterboxing is the process of adding black bars to the top and bottom of a movie or video after the entire picture has been scaled down to fit on a smaller screen that doesn't support the movie's large resolution. This happens because most movies or films are recorded in a widescreen format intended for theaters, which is wider than the format used by standard 4: 3 TV and 16: 9 HDTV.
The existence of many aspect ratios and formats for visual media has led to the creation of various methods to enable such media to be viewed on different devices with different aspect ratios compared to those used by the motion picture industry. Letterboxing is the most logical of these methods as it allows the entire picture to be displayed, albeit on a slightly smaller scale, as opposed to simply cropping the sides and leaving the center square picture when showing a widescreen movie in a 4: 3 aspect Ratio TV.
To fit the wide image on a smaller screen, it must be scaled down until both sides fit into the smaller aspect ratio. Since the image is a rectangle, it means that there are now blank areas at the top and bottom. The most logical way to address this problem is to make these areas black so that they can be largely ignored.