Certificate.

The BBFC (British Board of Film Classification) watch every film that is due to be screened in this country. They will then give it  certificate, U to 18, that indicates the age that a person has to be to see the film. The lower the rating, the more people will be able to watch the film as it will reopen to people of many ages. 

Often filmmakers will make cuts to their film to get it a lower rating. However it is important to remember that some film genres work better when they feature scenes that would only be shown in films that have a higher certificate. 

Horror/thriller films have previously been released baring an 18 certificate, but in recent years production companies have decided that it makes more sense to leave out certain aspects to get their film a 15 certificate. Meaning more people will be able to see it. I too have decided to make my film eligible for a 15 certificate.