In banking, when banks take a deposit from customers, they must keep a portion of it and not use it for lending, in case the customers need to withdraw their money. This portion is called the deposit reserve. The ratio between the deposit reserve and the total deposit amount is the reserve ratio. This kind of banking system is called the fractional reserve banking system.
What this implies is that banks basically have the ability to do what is called "money-printing". As an example: let's say a country's reserve ratio is 10%, then if you deposit $100 into the bank, the bank can lend out $90 to others, and charge them...