The Bank Book Balance screen displays:
The change is explained in terms of the total deposits and withdrawals processed for the bank.
A difference between the book balance and the G/L balance may indicate that there are unposted G/L batches. To ensure that the book balance and the G/L account are reconciled, you need to create any outstanding G/L batches (unless you use the option to create them during posting), and then post the batches in General Ledger.