Open is a casino term that has two different meanings, depending on the game you are playing.
If you are playing poker, the term will refer to the player who is the first one to make a bet in a hand. In a sense, they are opening a new round of betting, which is how the term was coined. Since the player on the button goes last, and the action is passed clockwise around the poker table, this means the player who’s paying the small blind is always the first one to act in a poker hand.
However, if you are playing keno, the term open is reserved for any point in time when the casino is open to accept new wagers for the upcoming keno draw.
No matter how you look at it, the term has a very similar meaning to that we encounter in everyday life outside casinos.