Initially Opening Balance for the Cash Book is entered in Setup / Cash Book Opening Balance. The balance is the balance as at the First Month of transaction entry. It will be updated as part of the Year-End Routine for subsequent financial years.
The Cash Book Opening Balance is any money to hand (but not in the bank) at the time of starting to use the system. It would typically be the total of cash, cheques etc. received from customers but not lodged by the first of the month you start to use the system.