Definition of bluff catcher
A bluff catcher is a hand strong enough to call a bet, but not strong enough to bet itself. A bluff catcher is a hand that beats a bluff. So the player who has bluff catcher on the river, for example, calls the bet assuming his opponent can bluff at a good frequency. If the opponent doesn't bet, the bluff catcher will be checked to go to abattage.
An example of bluff catcher
You have a pair of tens in hand. The flop falls 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 fails. 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 failed, 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 10's. The hand won't beat a hand that values as a three of a kind, two pair or top pair, but your hand will beat missed draws, such as the flush draw or even a straight.