The percentage charged for borrowing money, or paid for lending or saving it, usually expressed as an annual rate — the same underlying concept applies whether you're the one paying it or the one earning it.
Borrow money, and the interest rate is what it costs you for the privilege — a 6% rate on a loan means you'll pay back 6% more than you borrowed, roughly, each year it's outstanding. Save or invest money, and the interest rate flips to work for you instead — a savings account paying 4% grows your balance by roughly 4% a year, just for leaving it there.
Carrying high-interest debt (like a 20%+ credit card) while simultaneously investing for lower expected returns is usually a losing trade — the interest you're paying likely outpaces what your investments are earning. This is why paying off high-interest debt is often treated as a guaranteed, risk-free "return" in its own right.