This is a trap door, it's uses are quite expendable. In old times, or in cartoons, it was used in to toss people in the dungeon. Now they are used for things like wine cellars, and things like that. Generally, they have something that differs them from the flooring. Like, for example, they have the wood in the floor reversed on the trap door. Or they have a rope on the ground that you pull to open the door. These doors are not as common as they used to be.