Duration is the average capital tie-up period and represents the weighted average of the maturities of the future cash flows assigned to a security or portfolio.
So it is not a simple mean since each sum is weighted on the average (one due date) with the net present value of the corresponding cash flow calculated at the interest rate.
Since it is a weighted average of the maturities of the cash flows, duration represents a period.
The value of duration answers the question: If we were to convert all of the remaining cash flows of a security into a single cash flow (by converting a fixed-income security to a “zero coupon bond”), when would this security have to become due to have a term that corresponds to the average maturity of the original security "? In other words, “the value of duration” means the period of time it takes to recover the initial investment or the period of time one is exposed to interest rate risk.
It is no accident that the term duration is commonly used to measure a bond’s interest rate risk. Duration becomes shorter as the real maturity of the loan approaches. It increases as the coupon decreases.