"A hand" in card games means the cards that you're holding in your hand. You bluff your hand when you bid higher than it is reasonable. For example in poker, you have a high card - a ten (this isn't much) only and you bet all-in (all you have - that's much). It means you bluffed. If others knew what you have they would bet higher and you would lose your bet. But they don't know, so they can think that you must have a powerful hand to bet so high. They pass and you win. Generally to bluff means to misinform the opponent, to try to cheat them (but breaking no rules).