It provides a return in the form of fixed periodic payments and the eventual return of principal at maturity. Unlike a variable-income security, where payments change based on some underlying measure such as short-term interest rates, the payments of a fixed-income security are known in advance.
Example of a fixed-income security would be a 5% fixed-rate government bond where a $10,000 investment would result in an annual $500 payment until maturity when the investor would receive the $10,000 back. Generally, these types of assets offer a lower return on investment because they guarantee income.
It is a debt security similar to an informal debt instrument. When you purchase a bond you are lending a money to that entity who is issuers. In returns for the loan, the issuers promises to pay specified rate of interest during the life of the bond and to repay the face value of the bond when it matures or comes due.