The cashbook for cash at bank records all the transactions taking place at the bank i.e. the movements of the account held with the bank. The bank will send information relating to this account using a bank statement for the firm to compare.
Ideally, the records as per the bank and the cashbook should be the same and therefore the balance carried down in the cashbook should be the same as the balance carried down by the bank in the bank statement.
In practice however, this is not the case and the two (balance as per the bank and firm) are different. A bank reconciliation statement explains the difference between the balance at the bank as per the cashbook and balance at bank as per the bank statement.