Duration is an approximate measure of a bond's price sensitivity to changes in interest rates.
If a bond has a duration of 6 years, for example, its price will rise about 6% if its yield drops by a percentage point (100 basis points), and its price will fall by about 6% if its yield rises by that amount.
A bond's duration changes with time and as its price and yield change, however.
Why? Duration measures the time it takes to recover half the present value of all future cash flows from the bond. The discount rate for calculating the present value of the cash flows is the bond's yield. So as a bond's price and yield change, so does its duration.
For example, a bond with 10 years till maturity and a 7% coupon trading at par to yield 7% has a duration of 7.355 years. At a yield of 6% (price 107 14/32), its duration is 7.461 years. At a yield of 8% (price 93 7/32), its duration is 7.246 years.