Accrued interest is the interest that adds up (accrues) each day between coupon payments. If you sell a bond before it matures or buy a bond in the secondary market, you most likely will catch the bond between coupon payment dates. If you're selling, you're entitled to the price of the bond, plus the accrued interest that the bond has earned up to the sale date. The buyer compensates you for this portion of the coupon interest, which is generally handled by adding the amount to the contract price of the bond.
Use our Accrued Interest Calculator to figure out a bond's accrued interest.