You know when you watch a movie with your family, and sometimes there are some scenes that are too scary or too grown-up for you? Well, grown-ups used to have the same problem when they watched movies too! That's why in the UK, they made a system of film certificates that tell everyone how old you have to be to watch certain movies.
It all started a long time ago, in 1912, when cinema (that's what people used to call movies) was just starting to become popular. There were some people who wanted to make sure that movies were safe and appropriate for everyone, so they formed a group called the British Board of Film Censors.
This group watched movies and decided whether they were okay for everyone to watch, or if they should only be for grown-ups. They started off with just two ratings: "U" which meant that the movie was safe for everyone, and "A" which meant that it was only for adults.
As time went on, the world changed and movies became more complex. The ratings system needed to change too. In the 1950s, they introduced a new rating called "X" which meant that the movie was only for adults and had some pretty grown-up content in it.
But then people started getting confused, because they thought that "X" meant the same as in terms of movies we mean as "pornographic standards". So, in the 1980s, they changed "X" to "18", which still meant the movie was only for adults, but it wasn't as scary to people.
There are a few other ratings too, like "PG" which means "Parental Guidance" and "12A" which means that younger children can watch the movie as long as they're with an adult. Nowadays, the British Board of Film Classification (that's what we call it now) still watches movies and decides what rating they should have, to make sure that everyone can enjoy movies safely and with ease.