Okay kiddo, have you ever watched a movie or a show and noticed that there are some people who are not the main characters, but are still important and interesting? That's what we call supporting actors! They are like the sidekicks to the main character, and help them out or make the story more interesting.
Just like you have friends at school who are not always the leader but still help out and contribute to the fun, supporting actors are the same. They may not always be in the spotlight, but they are still important to the story and make it better.
Sometimes, supporting actors can be famous and well-known just like the main actors, but sometimes they are new and still learning. But either way, they all have an important job in the story and make it more fun to watch!