Generally 'fixed-income' returns refer to any investment vehicle that does not change the return payments over time or any other factor. (Some of these investment vehicles are backed by Federal and State Government and are some of the most stable investment options available.)
Fixed-income security is an investment vehicle that has regular payments over a period of time until the principal is withdrawn in it's entirety.
Since these have guaranteed returns, and are usually considered least risky, the rate of returns are also mediocre compared to other high-risk, variable, actively traded investment vehicles.
Some types of Bonds, Treasury Bills, COD etc,.. are some examples of Fixed-Income securities.
