To call the clock on someone at a poker table means that you’re limiting their time to act. If they don’t act within the allotted time, their hand will be declared dead.
For example, after a player takes five minutes when it’s their turn to act and they are still to make a decision, you decide to call the clock on them.
At this point, a floor person or a tournament director will come over and let the player know they have a certain amount of time (usually 60 seconds) to act. The countdown will start from the moment the floor person arrives, and the player will usually get a verbal warning once they are down to their last ten seconds.
If a player fails to act within the allotted time, their hand is folded.
Calling the clock is very uncommon in cash games, but it sometimes happens in tournaments. Any player at the table has the right to call a clock on any other player, and there are no steadfast rules as to when and why you can do it.
This is more a matter of poker etiquette. In most instances, a player will get a clock called on them only if they’re really taking way too long to act, significantly slowing down the game, especially in tournaments where blind levels are timed.