Liability in Matched Betting is the amount of money you need in your Betting Exchange in order to cover your Lay Bet.
If your lay stake is £5 with a liability of £30, you will need £30 or more in your betting exchange account if you want to be able to place your lay bet successfully.
When you place your Lay Bet at the exchange, your account balance will go down by the amount your liability was. This liability will then be 'locked away' from you until the bet settles (which is when the game or horse race ends).
If you had £50 in your betting exchange account and placed your £5 lay bet with a liability of £30, your betting exchange account balance would drop by £30 to be £20. The betting exchange has not taken this £30 from you but just 'locked it away' from you. You will be able to see the amount 'locked away' from you in your account balance section under the term 'exposure'.
When the game or race ends and your bets settle:
If your lay bet loses, you’ll lose your liability in the Betting Exchange, but win it back at the Bookmaker (as your 'winnings').
If your lay bet wins, your liability will be returned to your account plus you’ll also win your lay stake amount (minus any commission charged by the Betting Exchange).
You can read more about liability here:
https://www.outplayed.com/news/matched-betting/what-is-liability-in-matched-betting/