Accrued interest is the coupon interest a bond has earned since the last payment date but has not yet paid in cash. When you buy a bond between coupon dates, you pay the seller the clean price plus this accrued amount, and the sum is the dirty (full) price. On the next coupon date you receive the full coupon, which reimburses you for the accrued piece you prepaid. People often confuse clean price with the total cash you actually hand over at settlement.