The Triple Banana is an item found in the Mario Kart series. When you obtain this multi-item via an item box, you'll be able to shoot out three bananas either in front of you or behind you, compared to the one banana item. Once an opponent goes over the item with his vehicle, then he will rapidly spin around, thus slowing the car down. Another advantage about the item is that it will trail behind you, and if a player goes behind your cart, then he will hit the bananas and spin out of control.
It can also be useful if there is someone with a red or green shell, as the shells will hit the bananas instead of you.