Duration measures the interest rate risk on a bond. For those who are curious about some of the extra aspects of duration, you can read the following:
There is a a type of duration called Macauley, which basically is a weighted average of a bond's cash flows based on the timing of them. Individual interest payments get a smaller weight than the cash flow at maturity, which is the largest weight due the timing of it and the amount. The longer it takes an investor to get their money back, the longer the duration.
There is also a duration called modified, which allows a portfolio manager to estimate price changes on bonds when interest rates change. For example, if Bond A has a modified duration of 10 years and interest rates rise by 1%, the bond's price would be expected to fall by 10%.
You might also want to know that duration is always less than the years to maturity on a bond, expect for a zero coupon bond.