The secret door is a staple of role-playing games (especially Dungeons & Dragons).
A secret door is any kind of door that is not obviously a door. It may be a bookshelf that opens up when a certain book is removed. It could be the classic fireplace that rotates to open up a hidden room. Other popular secret doors include sections of walls that open up when pushed upon, and large paintings that are mounted on hinges and open upwards.
Almost any fantasy role-playing game module that you read will have several secret doors. But sadly, the designers almost never mention how they actually work. They just draw them on the map, and expect the gamemaster to use the games built in secret door rules (AD&D modules seem to be the worst at this, modules for games that do not have secret door rules tend to describe them much better).
Elves seem to be much better at locating secret doors than other races are. It is said that they are more in tune to the environment (although that is an after the fact explanation, the real story is that someone randomly made that rule in the first Dungeons & Dragons boxed set, and they came up with an explanation later).