Interest is how much it costs to borrow money. You can either earn interest when you lend money to somebody else or pay interest when you borrow money. Interest is the reason why debt is expensive. In addition the money you borrowed, you have to pay back an additional fee in exchange for using money that you didn’t have.
These posts discuss this in more detail:
Wealthy Maths: Nominal vs effective interest
Wealthy Maths: How to calculate real returns
Debt: Why debt costs money
Debt: Your DIY repayment plan
Podcast: The jargon buster episode