“Back” bets and the odds are always marked in blue, and ‘Lay’ bets are always marked in pink. “Back” betting is betting on an outcome To Happen while “Lay” betting allows you to play the role of the bookmaker. In a lay bet, you always stand to win the stake of the opposing bet; which is, betting on an outcome Not To Happen.
Liability is an important concept to understand when learning about laying bets. Think of yourself as the bookmaker. If you lose your bet, the bookmaker keeps your stake. If you win, then the bookmaker has to pay out your winnings. It can be a lot more than your original stake. This is the same as laying a bet. If the selection does win, you have to pay out the winnings. That potential payout is known as your liability. This is how much you need to pay out if you lose your lay bet.