The British Board of Film Classification (BBFC) is a group of grown-up people who watch movies and decide whether they are suitable for kids or not. They give each movie a rating like a grade in school. The grades are U, PG, 12A, 15, and 18.
The U rating means that the movie is for everyone and it's safe for little kids to watch. The PG rating means the movie might have some scary or intense parts that might not be good for really young kids, but it's okay for most kids with their parent's permission. The 12A rating means that the movie might have some more grown-up themes, like a little bit of violence or bad language, but it's still okay for kids who are 12 or older to watch, as long as they have an adult with them. The 15 rating means that the movie has even more grown-up themes, like lots of violence, bad language, or scary moments, and it's not suitable for kids under 15. Lastly, the 18 rating means the movie is only for grown-ups, and it might have really adult themes or really graphic violence or sex scenes.
The BBFC is kind of like a grown-up who decides when you're old enough to watch something more grown-up like Harry Potter or Star Wars. They make sure that movies are safe for you to watch and that you won't get scared or see anything you shouldn't.