A bond is like a loan.
It is a certificate (from a business or government) that says "You loaned us this amount, and we will pay it back with interest at such and such a date"
The bondholder (investor) lends money to the issuer.
Bonds can usually be bought and sold in the financial markets.
(Pictured is a bond for $1000 issued by the State of South Carolina in 1873.)