This is the action of discarding or folding one's hand without revealing the cards to the other players. When a player mucks their hand, they choose not to show their hole cards, conceding the hand and forfeiting any claim to the pot. It's a way to fold a hand discreetly without revealing the cards' value to opponents.
It can also refer to the pile of folded or discarded cards in the center of the table. These cards are considered out of play for the remainder of the hand. The dealer or the player responsible for managing the muck ensures that these cards are kept separate from the active deck to avoid confusion or accidental inclusion in future deals.