Hi, Punta.

Laying a horse means that you are being the bookie. In your case, you put up your $250 against the punter's $100. The punter naturally wants the horse to win whilst the bookie (you) wants it to lose.

If the horse does win the race, then you lose your $250 to the punter just like a bookie would, but if the horse does not win the race then you collect the punter's $100 (less the betfair charge of 5% maximum).