They are films whose theme produces fear, fright, panic viewers. They are usually about things paranormal, supernatural, ghosts, monsters or zombies.
It's the film of the horror genre you particularly can't see but which has a very broad public. They are films where the blood, tension, fear, suspense are the key to stand out from the rest. They are quite unpleasant and it should be much stomach to see them and be able to sleep then to loose leg.