Ratings

The BBFC defines the age certificate of 15 as :

Passed only for persons of fifteen years and over. No person apparently under the age of 15 years shall be admitted to any exhibition at which there is to be shown any film which has received a '15' certificate from the British Board of Film Classification.

The BBFC has strict guidelines on what should be in a film which has a certificate of 15:

Theme: No theme is prohibited, provided the treatment is appropriate to 15 year olds.
Language: There may be frequent use of strong language; the strongest terms are only rarely acceptable. Continued aggressive use of strong language and sexual abuse is unacceptable.
Nudity: There are no constraints on nudity in a non-sexual or educational context.
Sex: Sexual activity and nudity may be portrayed but without strong detail. The depiction of casual sex should be handled responsibly. There may be occasional strong verbal references to sexual behaviour.
Violence: Violence may be strong but may not dwell on the infliction of pain, and of injuries. Scenes of sexual violence must be discreet and brief.

Imitable techniques: Dangerous combat techniques such as ear claps, head-butts and blows to the neck are unlikely to be acceptable. There may be no emphasis on the use of easily accessible lethal weapons (in particular, knives).

"There are scenes of knife use in our film but the scenes........"

Horror: Sustained or detailed infliction of pain or injury is unacceptable.
Drugs: Drug taking may be shown but clear instructive detail is unacceptable. The film as a whole must not promote or encourage drug use.

15 means that the film, video or DVD is really only suitable for those aged 15 or above. Only people aged 15 or older are allowed to view a 15 rated film at the cinema or buy or rent a 15 rated DVD, video or game.