What is the comic relief?

The comic relief is a type of emotional or other tension resulting from a comic episode interposed in the midst of serious or tragic elements in a drama.

Why is it often used in horror film?

Because it's when you need comic relief characters will appear in fiction that is comic. It normally work when are in a dramatic moment.

Do you remember laughthing in horror films? Can you think of an example?

One of the times I laught more was when I watched Saw IV and the man who was supposedly dead was alive finally. But he stayed so many hours in the cold floor and when the victim dies he got upp.