Depending on the game, it may be a regular part of play or it may be considered cheating.
In poker, it most often refers to the discard pile into which players may throw their folded hands, and into which the dealer places burned cards.
[1] It also refers to when a player is folding his hand (face down) without saying anything.
a player may "muck" their cards in order to reinforce a bluff while preserving their image on the table.
A player conceals a card through sleight of hand, removing it from play so that it may later be inserted back into the game to the cheater's advantage.