When you invest in a bond, you are lending money to the bond issuer, generally for a specified time period. In return, the issuer is legally obliged to pay interest, or coupon, at a pre-agreed rate and to repay the original amount borrowed, principal, face value, nominal value, or par value on the repayment date (maturity).
Supply versus demand and the availability of money drive prices. Supply into an otherwise finite pool is set by the aggregate number of new shares issued less shares cancelled following buybacks or take-overs.
Demand is the variable to watch, and being a human sentiment coloured by fear and greed, it has many influences: