Interest is paid on an investment. For example, ordinarily, I can deposit my money with a bank and they will put that money to work and they'll pay me "interest" for the time that they have my money. They're keeping my money safe, but they're also profiting from it being with them and not with me. So they pay me, proportional to the current "interest rate" or "rate of interest". That rate is usually greater than 0. However, if the interest rate falls to below 0 then the bank doesn't pay me interest. Rather, I have to pay the bank interest for keeping my money safe. That's what it means when the interest rate (or interest rates) go negative.