Betting exchanges will allow you to lay a bet, which means you are betting for an outcome not to happen, as opposed for it to happen. For example, you can lay on a horse, so you are betting on that horse not to win.
What happens if you win a lay bet?
If you choose to lay a bet, you are betting on an outcome not to happen. For example, rather than backing Team A to win, you are laying them, so betting on Team A not to win. You can also lay outcomes in other sports such as horses not to win a race.