If you buy a bond, you sit back and collect the interest payments while waiting for the bond to reach maturity (the date the issuer has agreed to pay back the bond's face value).
However, you can also buy and sell bonds on the secondary market before the maturity date. After bonds are initially issued, their price can fluctuate. If you hold them to maturity the fluctuations generally won't matter since the face value to be paid at maturity won't change and the interest collected along the way will be stable.
But if you want to buy or sell a bond, keep in mind that the price you'll pay or receive is no longer the face value of the bond. The bond's susceptibility to changes in value is an important consideration when choosing your bonds.
Find out what role bonds can play in your portfolio. Talk to a financial advisor.