3-bet
A re-raise over the first raise (the blind counts as the first bet).
The cutoff opens to 2.5 big blinds with A-9 suited. The button looks down at K-Q suited and re-raises to 8 big blinds - the third bet in the sequence, after the blind and the open. The cutoff calls, the flop comes king-high, and the button is way ahead with top pair and the betting lead, instead of just calling and hoping to catch up.
A 3-bet does two jobs. With a strong hand it builds the pot early, while your opponent's range is still full of hands that fold to pressure - better than limping along and stacking someone only when the flop cooperates. With a weak hand it works as a bluff: opens from the cutoff and button run wide, and a big re-raise folds out marginal hands that would have flopped a piece against you. Good players mix the two - value hands like queens or ace-king, plus a few blockers or suited connectors - so the size alone doesn't tell anyone what they hold.
The beginner mistake is 3-betting only the top of the range: queens or better, ace-king. Sharp opponents read that inside a session. They fold everything and call or 4-bet only when they have you beat, so your monsters stop getting paid. Balance fixes it. Add a few bluffs and your value 3-bets get action instead of a fold every time.