The return earned when a security matures is known as the Yield to Maturity (YTM). It is a ratio for fixed-interest securities such as bonds and is expressed as a percentage, and it shows the average expected annual return, provided that the security is not sold before maturity.
It also helps when deciding whether it is worth selling the security ahead of time or holding it until maturity. In addition, this allows various bonds and their expected returns, for example, to be compared.
From the perspective of the buyer, the YTM allows you to estimate how much can still be earned with a bond until it matures.