It refers to a card in a player's hand that doesn't directly contribute to the rank of the primary hand but can break ties between players who have similar hands.
Kickers are crucial in determining the winner when multiple players have the same ranked hand, adding an extra level of hierarchy to hand comparisons.
Player A holds 2♥2♥ and the Player B holds 2♥2♥.
The board shows 2♥2♥2♥2♥2♥.
Both players have a pair of aces, so the one with the higher kicker (the next highest card) will win. Thus, Player A wins the hand as its kicker is 2♥.