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.
For example, the hand K-K-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, K-K-Q-3-2 would win (because its Q kicker outranks the 10), but K-K-10-4-3 would lose (because its 4 is outranked by the 5).