Older TVs used a screen size with a 4:3 ratio, the same ratio as most early movies.
Newer flat screen TVs use a wider ratio than the older sets, usually 16:9. As a result, older movies like "The Wizard of Oz," shot in 4:3 ratio, leave black bars on either side of those screens.
To counter the emergence of television in the 1950s, movies began using widescreen ratios, often 1.85:1 or even 2.35:1.
In the early days of home video, widescreen movies were shown on TV in the pan-and-scan format, which cropped part of the image and then blew the remainder up to fit the whole screen.
Film lovers and movie directors loudly protested the use of pan-and-scan, which they felt butchered the visual content of widescreen films. Some movies then began to be shown in their original widescreen format. In order to include the whole image on a TV screen with a different ratio, they used letterboxing, or black bars on the top and bottom of the screen.