The ratio of width to height of a computer monitor is 16:10, or 1.6:1, for example 1680x1050. 1680/1050 = 1.6
Now, most movies, the ratio is somewhere between 1.83 and 2.4 to 1, so for example, if the ratio was 2:1, then if it filled your screen horizontally, using all 1680 pixels, it would only take up 840 of the 1050 pixels vertically, hence letterboxing.