To back a team, horse or outcome is to bet on the selection to win. For LINE markets a Back bet refers to a SELL line. A SELL line will win if the outcome is LESS THAN the taken line (price)
LAY
To lay a team, horse, or outcome is to bet on the selection to lose. For LINE markets a Lay bet refers to a BUY line. A BUY line will win if the outcome is MORE THAN the taken line (price)