Device used to control human access to a certain area. Looks similar to the pairs of double doors seen at the entrance to supermarkets. You enter through the first door and it locks behind you. At this time you produce some form of identity verification or authentication (a badge, key, pass phrase spoken or key punched etc.). If all is well, the second door opens and you may enter. If there are doubts or problems, you end up locked in (usually with a nice "security is on the way" type announcement).
Man traps also prevent "piggy backing"; the practice of following an authorized person through a door to obtain illicit access.
A good example of a man trap is in the movie Sneakers, where the authentication was a voice print.