Plain English Meaning
A panel on top of the wing that pops up to kill some of the wing's lift on purpose. Pilots use it to come down faster, slow down, help bank the airplane, or plant the airplane firmly on the runway after landing.
Full Definition
A flight control surface mounted on the upper surface of the wing that, when raised, disrupts (spoils) the smooth airflow over the wing, reducing lift and increasing drag. Spoilers are used to steepen descent rates, slow the airplane, assist roll control, and on the ground to dump lift after touchdown so the wheels carry the airplane's full weight for effective braking.