A kicker is a card used to form part of a five-card poker hand, which in itself is not used to decide a hand’s ranking, but the “side card” or “kicker” is used to break ties, in situations where two or more players end up making the same best hand.
For example, on a board of 8-Q-8-4-2.
Player 1: K-Q
Player 2: Q-10
On the river, both players have two-pair, queens and eights, but Player 1 wins with a king kicker.
The kicker can be in the player’s hand, as demonstrated in the example above, or it can be one of the five community cards.
Dominated hands can be troublesome to play because even when they make top pair hands due to kicker issues they will frequently lose to hands with better kickers.