Okay kiddo, so imagine you want to watch a movie that's not out on DVD yet, but it's only showing in theaters. Someone might secretly bring a video camera into the theater and record the movie while it's playing on the big screen.
That video recording is called a "cam" or "bootleg." It's not a good quality video because you can sometimes see the heads of people in front of the camera or hear them rustling popcorn bags. It's also illegal because the person recording it didn't have permission to do so, and it takes away money from the people who made the movie.
So remember, it's important to wait until a movie comes out on DVD or legitimate streaming services so we can support the people who worked hard to make it.