How much you will have in your balance after any active studies have been completed. That is, if you publish a study with an overall cost of £50, your available balance will be your total balance minus £50. However, the £50 won't be deducted from your total balance until you approve the submissions you receive from this study.
Therefore, the difference between your total and available balance is reserved for any active studies you have running on your account.
To increase places for a study, or to publish a new one, your available balance must be high enough to cover the costs of the additional participants. If you Stop an active study before it reaches the number of Maximum Submissions, any unspent funds from the participants you did not recruit will be restored to your available balance for use elsewhere.