How are profit and liability worked out?

Back bet: your stake is at risk. If the selection wins, your profit is stake × (odds − 1).
Example: a 100 back at 2.50 wins 150 profit; if it loses you lose 100.

Lay bet: you accept the backer's stake. Your liability (what you can lose) is stake × (odds − 1).
Example: a 100 lay at 2.50 has a liability of 150; if the selection does not win you keep the 100 stake.

Your liability is held from your balance until the market is settled. The bet slip always shows the profit and liability before you confirm.