Check-raise
A check-raise is checking when the action reaches you, then raising after a player behind you bets in the same betting round.
It is only possible out of position, because someone has to act after you for there to be a bet to raise. That is part of why it exists: it is one of the few ways to get more money in from the seat that acts first.
It is used with the strongest hands and with the weakest ones, for the same reason. A hand that wants a bigger pot and a hand that wants the pot right now both benefit, and using it with only one of the two makes it easy to read.
an example
your cards
board
You flopped three eights out of position. Betting into the raiser often wins a small pot. Checking, letting them bet, then raising builds a large one against a hand that has already committed money and may well continue.
This example is made up. The real one is in your own hand history: find check-raise in your own hands and see how often it came up and what it cost you.
related terms
C-bet
A continuation bet, or c-bet, is a bet on the flop made by the player who raised last before the flop.
Slow play
To slow play is to check or call with a strong hand instead of betting it, hoping the opponent keeps betting or catches ...
Semi-bluff
A semi-bluff is a bet or raise with a hand that is probably not the best right now but can still improve, so it can win ...
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Knowing the word is not the same as seeing it.
Forty hands against a bot, no account, and your own hand history at the end.