Bet liability refers to the amount you are risking when placing a bet. This amount will be deducted from your balance should your bet lose.
The liabilty of a back bet is the value of the stake.
When you place a bet, the liability is instantly deducted from your available balance. If the bet is cancelled the liability will be returned.
If you have different bets placed on the same event, your risk is decreased and your overall liability is reduced.