A letterbox is a rectangular hole in a door or a small box at the entrance to a building into which letters and small parcels are delivered.
ADJ
（电视或计算机显示）信箱格式的，上下黑边的，宽屏的
If something is displayed on a television or computer screen in letterbox format, it is displayed across the middle of the screen with dark bands at the top and bottom of the screen.