Have you ever seen a movie or TV show where one actor plays two different characters who look exactly the same? They might use a body double to help make it look like there are two different people on screen.
A body double is someone who looks very similar to the actor playing the character. They might have a similar height, weight, hair color, and face shape. When the scene requires the actor to be in two different places at the same time, or if the scene is too dangerous for the actor to do themselves, a body double steps in.
For example, let's say the character needs to jump off a building but it's too dangerous for the actor to do it. The body double would do the jump instead, but you wouldn't be able to tell the difference because they look so similar.
Body doubles are used to make movies and TV shows more believable and keep the actors safe. It's kind of like having a helper who looks just like you and can do some of the harder stuff for you.