The muck is the pile of dead and discarded cards in front of the dealer. It can be also used as a verb, in which case “mucking” refers to the action of folding your hand and discarding your cards in the direction of the pile. You can muck your cards during any of the betting rounds, as well as at showdown if you don’t want to reveal your hand to your opponents.
Muck Poker Example:
You have a K K hand, and the board reads K Q J 8 5 after the river. After you and your opponent check, they reveal an A T hand, making a straight. Knowing they have you beat, you accept the defeat and muck your cards, deciding not to show them.