A kicker, also called a side card, is a card in a poker hand that does not itself take part in determining the rank of the hand, but that may be used to break ties between hands of the same rank.[1][2] For example, the hand Q-Q-10-5-2 is ranked as a pair of queens. The 10, 5, and 2 are kickers. This hand would defeat any hand with no pair, or with a lower-ranking pair, and lose to any higher-ranking hand. But the kickers can be used to break ties between other hands that also have a pair of queens. For example, Q-Q-K-3-2 would win (because its K kicker outranks the 10), but Q-Q-10-4-3 would lose (because its 4 is outranked by the 5).
Kickers in draw poker
The term is also used in draw poker to denote an unmatched card (often an ace) retained by a player during the draw in the hope that either it will be paired on the draw, or else play as a kicker (in the first sense) on the showdown. A kicker may also be retained in order to deceive an opponent, for example, to represent a three-of-a-kind when the player has only a pair.
Kickers in Texas hold 'em
Kickers take on special importance in Texas hold 'em, because a common winning hand is one card in a player's hand matched with a card on the board, while the player's second card acts as a kicker. For example, if one player holds A-8, a second player holds A-7, and the board is A-K-6-5-4, the player with the A-8 will outkick the player with the A-7, since A-8's best hand is A-A-K-8-6, while the A-7's hand is A-A-K-7-6. However, if the board held A-K-Q-J-3, the players would tie, because both would play the hand A-A-K-Q-J; in this case it is said that the players' kickers "don't play", or that the "kicker on the board plays". In this case, there would be a split pot.