To lay a bet is to back something not to happen. For example, to lay Manchester United 馃崑 to win their match is to back them NOT to win. If you were to lay them, you would win 馃崑 your bet if they either lost or drew their match.
With lay betting, you are betting on any other possibility. For example, if you place a lay bet on Team 馃崑 A, you will make a profit if Team A loses or the game ends in a draw.