How it works
In an old town in Italy, a banker sat at a long bench. A bench was called a banco, and that is where our word bank comes from.
Job one is keeping coins safe. The banker locks them in a strong box, writes the amount in a big book and gives a paper slip.
Job two is lending. To borrow means to take coins now and give them back later.
The extra coin Dario paid back is called interest. It is the price of borrowing, and the banker's pay.