What is a Weak Kicker in Poker?

Holding a weak kicker means that while one card made a pair, your other card is relatively weak, and someone could have a better kicker. 

For example, let’s say you have Ad 3d on a board of As 9h 7c. While you have top pair, you only have a 3 kicker, meaning that if your opponent has an ace, they likely have a better kicker than you.