A bond's coupon is fixed when it's issued. If interest rates then rise, new bonds pay more, so an existing bond is only worth buying at a lower price. The yield to maturity is the return you'd get from buying at today's price and holding to the end, coupons and repayment included. It moves opposite to the price: when prices fall, yields rise.
How far the price moves depends on duration, measured in years. As a rough rule, a duration of 7 means about a 7% price fall for each percentage-point rise in rates. Long-dated bonds have long durations, so they carry the most interest rate risk: in 2022, as rates rose quickly, long-dated gilts lost more than a third of their value. Holding an individual bond to maturity still returns par if the borrower pays, but a bond sold before then is sold at whatever the market will pay.
An illustrative rule of thumb. The longer the duration, the harder rate rises hit the price.
Educational content only. It is not personal financial advice.