A cooler is a hand lost not because of a mistake but rather because of the way the cards came out; a player with a strong hand loses to another player with a stronger hand. The losing player usually loses a lot of money and there is no way to avoid it with correct play.
An example of a cooler is when two players in Texas Hold'em both have a high pair and each hit their three of a kind on the flop. It's not a bad beat, just bad luck that the cards were such that correct play on the part of both players leads to one losing terribly. Both players had very strong hands.