An interest rate is the price of using someone else's money, expressed as a percentage of the amount borrowed over a stated period — usually a year.
Two rates are only comparable when the period and the compounding match, which is where most confusion starts: a "2% monthly" rate is not 24% a year, it is closer to 27% once compounding is counted. When comparing loans, the rate alone is also incomplete, because mandatory fees change the real cost. That is what APR exists to capture.