This field shows the accumulated balance of the bank reconciliation, which consists of the amount in the Balance Last Statement field plus the total balance in the Statement Amount field.
Note that you are not allowed to post the reconciliation lines unless the amount in this field and the amount in the Statement Ending Balance field agree.
If the two balances do not agree, you can adjust the amounts in the Statement Amount field on the reconciliation lines or you can insert a new line with the amount of the difference and post it to the appropriate journal. Now you will be able to post the bank reconciliation.