Where it all starts
Money & Time
A dollar isn't frozen in time. Park it somewhere that pays interest and it grows; borrow it and it costs you. That price of using money over time is what 'interest' means.
Interest is just rent on money. When you save, the bank rents your money and pays you. When you borrow, you rent theirs and pay them. Same idea, opposite direction.
Interest is rent on money — you can be the one collecting it or the one paying it.