Mantraps are physical security barriers that are setup as small rooms with a entry door on one wall and an exit door on the opposite well. One of the mantrap doors cannot be unlocked and opened until the opposite door has been closed and locked.
Mantraps are most often used in physical security to separate non-secure areas from secure areas and prevent unauthorized access when someone is trying to tailgate. They can also be found in high tech manufacturing to provide entry and exit chambers for clean rooms. Mantraps are used alot in the corporate world.