The liquidity is the amount of money available for you to bet (back/lay) at the corresponding odds on the corresponding selection. The liquidity that you see available at each odds increment can be a combination of one or more different users' unmatched bets. You can bet all or part of that amount. If you bet more than the amount shown, the system will place the remaining stake as an unmatched bet available for others to take.
The liquidity is displayed on the odds buttons of each selection on which you can bet.
You will see the liquidity in the currency of your account if you are logged in and in GBP (£) if you are logged out.