There are some interest rates that are open: open to the big, scary world of supply and demand. More supply and less demand? Open-market rates go down. Less supply and more demand? Open-market rates go up. All peer-to-peer investing markets (“secondary markets”), like your typical stock market, are places where open-market rates are roaming free, like happy cows in a field of green.
All kinds of bonds, CDs, and after-market stocks are different breeds of these happy cows, changing their interest rates according to which way the winds of supply and demand are blowing that day. Oh, the fresh air of open-market rates.
There are interest rates that aren’t so free to roam, mainly the ones set by the Federal Reserve. The Fed sets the discount rate and fed funds rate, which both affect the lending rates of banks. By toying with interest rates, the Fed can try to affect the economy one way or the other (re: monetary policy).