3-bet
Also called three-bet
A 3-bet is the third bet in a betting round: a re-raise made against someone who already raised before you, putting in more chips to take control of the hand.
The name comes from counting the rounds of aggression preflop. The big blind is the first "bet," the first raise is the second, and your re-raise on top of that raise is the third, so it's called a 3-bet. It applies after the flop too, where the original bet counts as the first and a re-raise of a raise becomes the 3-bet.
Most often you'll hear it preflop. Someone opens with a raise, and instead of just calling along (a cold call) or folding, you raise again to a bigger size. That re-raise is the 3-bet, and it usually announces a strong hand or an aggressive play to win the chips right there.
As a beginner, you can keep it simple at first by 3-betting your premium hands like big pairs and ace-king for value, meaning you want callers. Sizing matters too: a typical 3-bet is roughly three to four times the original raise, a bit larger when you're acting out of position. Over time you can add the occasional bluff 3-bet, but there's no rush.
Example
It folds to the cutoff, who raises to 3 big blinds. You're on the button with A-K and re-raise to 9 big blinds; that re-raise is your 3-bet, and the cutoff now has to decide whether to fold, call, or 4-bet.