A mantrap, in modern world, means a small room that is encased or made-of metal, and contains two or more doors. The principal rule is that both doors cannot be open at the same time.
If a person needs to access a facility, he must access it through a mantrap. He might use a keycard to open the first door, then enter the mantrap and close the door behind him, and only then he can enter his personal PIN (for an example, could be biometric as well) which, if entered correctly, will open the second door.
Failure to enter the PIN code, or trying to force either of the doors open, will trigger an alarm. An intercom system will be activated, and an announcer will notify that a guard-team has been dispatched to clear the solution, and apprehend the suspect to the police.
Such "boxes" usually also contain tailgate
-prevention technologies, so that only one person can enter at a time.
In history, mantraps could also cause deadly forces at the intruder, possibly by using a sleeping-gas, impaling spikes or emitting a high-energy noise. Such are illegal today.
Mantraps are very scary to the people who rarely use them. The smell, the sounds... And when you enter a mantrap, the silence is somewhat disturbing. There's a set amount of time in which you need to enter your code, or an alarm will be triggered. This could be 10 seconds or less.
Most advanced mantraps enforce tailgating-prevention by taking it to the next level, with pressure-sensitive plates on the floor. What this means is that when you show your keycard at the first door, the mantrap recognises who you are, and fetches your last weight from the databases. When you enter the mantrap, the pressure sensitive plates at the floor will measure your weight, and if it's too far off from the last weight, it'll trigger an alarm. This effectively prevents two people from going in together, but if you're carrying something heavy, you cannot pass.
Metal detectors can also be included, so that if you carry ANYTHING metal, the second door won't open. A guard-team is dispatched to investigate what you are carrying in or out.
The main hacker in the movie Sneakers ('92) went through a mantrap with two fortified glass doors. The second door was opened by speaking a displayed message and using a keycard. The message was: "Hi. My name is XXXXXX. My voice is my passport. Please verify me."