According to Britannica.com, a bank is:
Banks are critical to our economy. The primary function of banks is to put their account holders' money to use by lending it out to others who can then use it to buy homes, businesses, send kids to college...
When you deposit your money in the bank, your money goes into a big pool of money along with everyone else's, and your account is credited with the amount of your deposit. When you write checks or make withdrawals, that amount is deducted from your account balance. Interest you earn on your balance is also added to your account.
Banks create money in the economy by making loans. The amount of money that banks can lend is directly affected by the reserve requirement set by the Federal Reserve. The reserve requirement is currently 3 percent to 10 percent of a bank's total deposits. This amount can be held either in cash on hand or in the bank's reserve account with the Fed. To see how this affects the economy, think about it like this. When a bank gets a deposit of $100, assuming a reserve requirement of 10 percent, the bank can then lend out $90. That $90 goes back into the economy, purchasing goods or services, and usually ends up deposited in another bank. That bank can then lend out $81 of that $90 deposit, and that $81 goes into the economy to purchase goods or services and ultimately is deposited into another bank that proceeds to lend out a percentage of it.
In this way, money grows and flows throughout the community in a much greater amount than physically exists. That $100 makes a much larger ripple in the economy than you may realize!