Bluff can mean a high cliff, or it can describe a person who is abrupt in manner. The most common usage of bluff is as a verb meaning to pretend. If you bluff at cards, you are pretending to have a better hand than you do.
Many a good crime story ends with a bluff—"I've already contacted the police and they're on their way," the detective will say when that is not at all the case. The expression "calling your bluff" comes from poker, where you "call" a game to see someone's actual cards. Calling someone's bluff means forcing them to admit the truth.