A duration of seven means that if yields rise one percentage point, the price falls roughly seven per cent. It is the single number that turns an interest rate forecast into an expected profit or loss.
Longer maturities and lower coupons both extend duration, because more of the return arrives further in the future. A thirty-year zero-coupon bond is the most rate-sensitive instrument in the market, and a short-dated high-coupon bond among the least.
It is the risk that surprised people in 2022, when a rapid tightening cycle produced double-digit losses in supposedly safe long-dated government bonds, and again in 2023 when US regional banks discovered what duration meant for a portfolio they had to sell.