I suspect you mean some kind of automatic gadget which makes doors open for the right people?
It probably has a transponder chip in it. Near the door is a radio transmitter sending a coded message. The transponder chip uses a tiny bit of the radio energy to respond with a very weak coded radio message, detectable by a receiver nearby. If this code is correct, the door opens.
The anti-theft gadgets in shops work in a somewhat similar way, and so do the identification chips implanted in pets.
It could perhaps also be a more sophisticated circuit, with a battery and programmable code.