I believe the answer is: An investor makes money by earning interest.
When an agreement to borrow a certain amount of money is created, the borrower would receive the requested amount of money at the time that specified in the contract. (most likely used as capital to fund their business or to buy necessities).
In return, the borrower has to payback the money to the lender gradually, in a higher amount than he borrowed. The difference between the money borrowed and the money paid back is what's called as interest by the investors.
