In casinos and card rooms, this marker is used to indicate players who have missed their blind. When the player returns, they must post a blind to get back into the game.
A mandatory bet that is not posted
When a player sits out and misses a blind, in order to receive a hand, he/she must post both blinds or wait a turn to take the big blind.
A required bet that is not posted when it is your turn to do so.