There are two ways to make money by investing in bonds. The first is to hold those bonds until their maturity date and collect interest payments on them. Bond interest is usually paid twice a year.
The second way to profit from bonds is to sell them at a price that's higher than you initially paid.
For example, if you buy $10,000 worth of bonds at face value -- meaning you paid $10,000 -- and then sell them for $11,000 when their market value increases, you can pocket the $1,000 difference.
Bond prices can rise for two main reasons. If the borrower's credit risk profile improves so that they’re more likely to be able to repay the bond at maturity, then the price of the bond typically rises. Also, if prevailing interest rates on newly issued bonds go down, then the value of an existing bond at a higher rate goes up.