Interest rate changes are among the most significant factors affecting bond return.
To find out why, we need to start with the bond's coupon. This is the interest the bond pays out. How does that original coupon rate get established? One of the key determinants is the federal funds rate, which is the prevailing interest rate that banks with excess reserves at a Federal Reserve district bank charge other banks that need overnight loans. The Federal Reserve (or "the Fed") sets a target for the federal funds rate and maintains that target interest rate by buying and selling U.S. Treasury securities.
When the Fed buys securities, bank reserves rise, and the federal funds rate tends to fall. When the Fed sells securities, bank reserves fall, and the federal funds rate tends to rise. While the Fed doesn't directly control this rate, it effectively controls it through the buying and selling of securities. The federal funds rate, in turn, influences interest rates throughout the country, including bond coupon rates.
Another rate that heavily influences a bond's coupon is the Federal Reserve Discount Rate, which is the rate at which member banks may borrow short-term funds from a Federal Reserve Bank. The Federal Reserve Board directly controls this rate. Say the Federal Reserve Board raises the discount rate by one-half of a percent. The next time the U.S. Treasury holds an auction for new Treasury bonds, it will quite likely price its securities to reflect the higher interest rate.
What happens to the Treasury bonds you bought a couple of months ago at the lower interest rate? They're not as attractive. If you want to sell them, you'll need to discount their price to a level that equals the coupon of all the new bonds just issued at the higher rate. In short, you'd have to sell your bonds at a discount.
It works the other way, too. Say you bought a $1,000 bond with a 6 percent coupon a few years ago and decided to sell it three years later to pay for a trip to visit your ailing grandfather, except now, interest rates are at 4 percent. This bond is now quite attractive compared to other bonds out there, and you would be able to sell it at a premium.