Complex sentence: Fractional-reserve banking is a type of banking whereby the bank does not retain all of a customer's deposits within the bank and customers leaving deposits with the bank are unsecured creditors of a private firm.
Split into simpler sentences: Fractional-reserve banking is a type of banking whereby the bank is under no obligation to retain all of a customer's deposits within the bank. Customers leaving deposits with the bank are unsecured creditors of a private firm.