Definition of kicker
A kicker is a sidekick. The sidekick is used to break the pot when two players have the same pair. For example, if two players have 2-7-A-3-8 in the showdown, the player with AK will have a pair of aces and a king kicker and the player with AQ will have a pair of aces and a queen kicker. The player with AK wins the pot with his sidekick (kicker). We always consider 5 cards in a Hold'em hand. If our two players previously with AK and AQ were now on a 5-5-6-6-7 board, it looks like each player has 2 pairs (a pair of 5's and a pair of 6's) with an ace kicker for the fifth card. Now the king and queen would not be considered.
The importance of kickers
One of the mistakes that beginner players make when they start out is not to consider kickers. This is a mistake that costs them a lot of money. If you have a hand like A2 on A-5-5 and your opponent bets, it is okay to call. If the turn is an 8 and he bets again, you can think about it, but let's say you call again. And if the river is a 9 and he still bets, it's very likely that our opponent has a better kicker than your 2 here and calling all 3 bets with a single pair of aces and a bad kicker is certainly losing versus the majority of villains.