The price of money, before fees.
An interest rate is the percentage a lender charges, or a bank pays, on a balance over a year. It excludes fees, which is what separates it from APR on a loan and from APY on savings.
Fixed rates stay put for the life of the loan. Variable rates move with a benchmark such as prime or the Fed funds rate.
The Federal Reserve's policy rate is 3.63% today, and nearly every other rate on this page is priced off it.
See the fed funds page