Check-raise
A check-raise is when you pass the action by checking, then raise after an opponent bets in that same round, instead of just calling their bet.
It's a two-step move inside a single betting round. First you check, handing the action to someone behind you. When they bet, you come back over the top with a raise rather than folding or flat-calling. Because you showed weakness first, the raise often catches opponents off guard, which is why it's considered a trapping or deceptive play.
It usually comes up when you're acting before the bettor, most often from the blinds or otherwise out of position. A common spot is checking to the player who took the lead before the flop, letting them fire a continuation bet, and then raising once they've put chips in. You can do it for value with a strong hand to build the pot, or as a bluff to pressure a hand that probably can't continue.
For a beginner, the safest version is the value check-raise: check a genuinely strong made hand, let an aggressive opponent bet into you, then raise to grow the pot. Bluff check-raises work better once you can read when an opponent is likely just stabbing at the pot with a weak holding. Lean on it occasionally rather than every hand, since checking strong hands too often makes you easy to read.
Example
You defend the big blind with 7h 6h and the flop comes 8h 5c 2h, giving you an open-ended straight draw plus a flush draw. You check, the preflop raiser c-bets 4 chips into a 9-chip pot, and you check-raise to 13 chips, applying pressure while you still have plenty of outs to improve.