Those having extra cash open a bank account in their name and deposit the surplus money there. Their money is safe with the banks and they get some interest rate on the amount they have deposited.
Out of the total money deposited with the banks, 15% of it is kept as minimum cash balance to pay to the depositors who might come to withdraw money from the bank on any given day. Rest of the amount is used to extend loan to the borrowers. This helps other people to run and expand their business which will increase the individual income and finally the country’s income.