Definition of bluff catcher
A bluff catcher is a hand strong enough to call a bet, but not strong enough to bet itself. The bluff catcher is a hand that beats a bluff. So the player who has the bluff catcher on the river, for example, calls the bet assuming that his opponent can bluff at a good rate. If the opponent doesn't bet, the bluff catcher will check to the showdown.
An example of a bluff catcher
You have a pair of tens in your hand. The flop comes 5-8-J with a flush draw and your opponent bets. You call. The turn is another 8. Your opponent still bets and you call. On the river, you notice that the flush draw misses. It is an ace. Your opponent bets big. In this situation, against an aggressive player, your hand is a perfect bluff catcher. Since the flush draw missed, your opponent could take advantage of the ace on the river to bluff with a hand like KQ that would have the flush draw. So you call with your pair of tens. The hand won't beat a value-betting hand like three of a kind, two pair or top pair, but your hand will beat failed draws, like the flush draw or even the straight draw.