A cooler refers to a situation where two players remain in a hand both with very strong hands. Neither player is likely to fold as they have a strong hand. The player with the weaker hand has the cooler.

For example if the flop in a game of hold'em comes up:

One player holds A♥ A♣ and the other player 7♠ 7♦. The player with aces, who now has a full house, has a great hand. There is less than a 1% chance that a player with a pocket pair flops four of a kind. Obviously the player with quads will call any bet, yet the player with a full house will also call or raise since he has the second nut hand.