4-bet
The re-raise over a 3-bet. Usually says premium — or a bold bluff.
Blinds are 1 and 2. The cutoff opens to 6 with pocket jacks. The button 3-bets to 20 with ace-queen. The small blind wakes up with pocket aces and 4-bets to 46. It folds back to the button, who has a decision worth the session: put nearly a quarter of a 200 stack in with ace-queen, or let it go. That raise to 46 is the 4-bet — the second re-raise of the hand, after an open and a 3-bet.
The 4-bet is where the pot stops being cheap. Facing one, folding usually costs less than finding out whether ace-queen is good against a range that's mostly aces and kings. Good players also 4-bet as a bluff, shoving hands like ace-five suited: the ace in their hand leaves fewer aces and ace-kings for the opponent to call with, so a raise that used to mean a monster sometimes doesn't. Now nobody can fold everything but aces to it.
The beginner mistake is 4-betting face up — only aces and kings. The raise gets easy to read and easy to fold to, so nobody pays off the monsters. The opposite leak is never 4-betting at all, flatting 3-bets out of position with a hand that wanted to isolate and handing aggressive players the lead. The last one is sizing too small: a min-raise over a 3-bet gives the opener a cheap, profitable call with a hand that should have folded to real pressure.