A bond is a bit like a term deposit at a bank, but it's effectively a loan you make to a company or government which they promise to pay back in full on a specific date. Until then, you receive regular interest at a fixed or agreed rate.
A city, for example, may issue bonds to raise money to build a bridge and pay an agreed amount of interest until the bonds' "maturity date". If you want your money back before the maturity date, you can sell your bond on a secondary market, or you can buy a bond on that market from others.
Bonds are income assets and can also be called "fixed interest". Here's more on the basics of bonds. This site features regulated bond offers, including more complex "debt securities".