An antechamber is an entryway or a small room that leads into a larger one. If you visit a friend who lives in a mansion, her butler may ask you to wait in the antechamber while he summons her.
You're most likely to come across an antechamber in a very grand building or home — in most houses, a similar room would probably be called a "foyer" or a "hall." Sometimes the area where you wait before entering a museum or office is called an antechamber, but it's more often just a "waiting room." Palaces and crypts and pyramids often have antechambers. The word comes from the French antichambre.