I am sure you have heard about bank interest, but never have been told what is that exactly. I would say that INTEREST is how much is paid for the use of money (expressed as a percent or an amount). For further information, click on here (FOCUS ON SIMPLE INTEREST ONLY).
Have you ever wondered how a real bank works? See that video if you are interested:
If you want to practice doing exercises, have a look here.
This is the main vocabulary you should know for that unit:
LOAN, CREDIT: préstamo
RATE OF INTEREST: Tipo de interés
BORROW: Tomar/pedir dinero prestado
LEND: Prestar dinero