The bar is the raised ridge in the middle of the backgammon board where hit checkers are placed. When one of your checkers is hit while it is a blot, it goes to the bar and is temporarily out of play. Your very next task is to re-enter that checker into your opponent's home board before you can move anything else. If you have a checker on the bar and cannot enter it, your whole turn is wasted. Because being sent to the bar sets you back so much, controlling the bar situation, both avoiding it and forcing it on your opponent, is a key part of the game.