When you place an order, you are essentially stating "I would like to buy this position". When you do this, BettorEdge puts aside the amount of money you wagered so that if you get matched with another bettor, we know that you have the funds to cover it. When an order expires, it is no longer an open order and those funds are now available for you.
By default, the expiration is at the start of the game, however you can adjust it.