A fixed-income asset of an investor is one which provides fixed periodic payments, and at the end of its maturity (holding period), repayment of the principal sum invested.
Sovereign bonds, company bonds
For the issuing company, a fixed-income product is a form of debt, and it is obliged to pay its debt before rewarding its equity holders (of common shares, for example). For the investor, this is lower risk as the payment obligation supersedes that on equity shares.
A fixed income product allows governments, corporations, and municipalities to take debt from the public to fund their projects.