Have you ever gone to see a movie in a big movie theater with lots of other people? When you buy a ticket for the movie, that's called contributing to the box office. It's like a big collection of all the money that people spend to see that movie.
The reason that people keep track of the box office is because it helps the movie industry figure out which movies are popular and which ones aren't. If lots of people go to see a movie and buy tickets, that means it's a hit! But if not very many people go see it, that means it's not doing very well.
Sometimes the box office is measured in terms of how much money a movie makes. So if lots of people go see a movie and each of them pays, say, $10 for a ticket, that means the movie has made $10 for each of those people who went to see it.
So the box office is just a way of keeping track of how many people are going to see a movie and how much money the movie is making. It can help the movie industry decide which movies to make more of and which ones maybe didn't do so well.