Hey kiddo! Have you ever watched a movie or a TV show that had black bars at the top and bottom of the screen? That's called "letterboxing"!
When people make movies or shows, they usually film them in a certain shape. This shape is called the "aspect ratio". Some movies are filmed in a wider shape, and some are filmed in a more square shape.
When a movie or show is filmed in a wider shape, and then it's shown on a screen or TV that has a more square shape, some of the picture won't fit. So instead of squishing the picture, they put black bars at the top and bottom so the whole picture can fit on the screen.
Letterboxing means more of the original picture is kept, even though it doesn't fit the TV screen perfectly. This way, we get to see more of what was originally filmed!