What are genre conventions?

Conventions is a way of something which is accepted by the audience for a certain type of category such as  conventions of comedy genre is funny punchlines. Or a convention of a horror film is scary music; it is the way we recognize what type of film something will be. A good trailer will show conventions of a genre that the film is. For example, the trailer for saw 7 starts with eery music and dark screen with bold writing.