A check raise is defined a play made post flop from out of position. It is when a player checks and then re-raises a post flop bet.
It is often associated with a play made with a strong hand as to check and raise is more expensive than betting. By check-raising one is investing more than they need to, something players would not expect with a bluff hand.
Example: Player A raises to $10 and Player B calls from the BB. Player B checks on a Ac As 2c flop. Player A bets $20 and Player B raises to $65. This is known as a check raise.