In 75 Ball Bingo, each bingo card has 25 spaces, or squares, each containing a number, except for the centre square. The centre square does not have a number assigned to it except for "free space" or "free square". This square is considered already filled or daubed and is usually marked "free", much like a Joker or a Wild square. Every bingo card has one and it counts towards your winning pattern.
Using the free space in bingo halls or online bingo is a great way to increase your odds of winning. Think of it... when you use the free space in diagonal bingo you only need to get four numbers and you've won! Although everyone knows this I think, that's probably why when the free space is involved you'll see more multiple winners than usual.