Answer Description
A mantrap is a device designed to physically trap a person in an enclosed area. Typically, this is a set of doors that cannot be opened simultaneously. The idea is that this will help to prevent tailgating, as the intruder will be momentarily trapped inside the same room as the authorized user.
Wikipedia
A mantrap, air lock, sally port or access control vestibule is a physical security access control system comprising a small space with two sets of interlocking doors, such that the first set of doors must close before the second set opens Airlocks have a very similar design, allowing free ingress and egress while also restricting airflow In a manual mantrap, a guard locks and unlocks each door in sequence An intercom and/or video camera are often used to allow the guard to control the trap from a remote location In an automatic mantrap, identification may be required for each door, sometimes even different measures for each door
Mantrap_(access_control) - Wikipedia, the free encyclopedia