In poker, to call the clock means to put a player to act on a set amount of time to make their decision. If they fail to make the decision within the given time, their hand is automatically declared dead.
When someone calls a clock on another player, a floor person or a tournament director will come to the table and let the player know how much time they have to act (usually 60 seconds, but sometimes it can even be 30).
As the time runs out, the floor person will let the player know how much time they have left, usually counting down the last 10 seconds. If they fail to make a decision before the countdown reaches zero, their hand is dead.
There are no fixed poker rules defining when a clock can be called. However, it is poker etiquette to only do this after the player has had plenty of time to act (at least two or three minutes). Calling the clock is somewhat common in tournaments, but it is very rarely done in cash games, even if a player tanks a lot.
