in a poker game to tag the 3rd player as being away from the table. When there is a list of players waiting to play, getting tagged like this is supposed to cause the 'floor' to come over and pick you up, i.e., remove your chips and give the seat to another player.

This is very simplified, as the overall subject can fill a Chapter (or more) in a text book on running a poker room! However, this is an up-scale version of a "3rd Man Walking" button.

Jim