On large passenger aircraft, there are flat panels on TOP of the wing that pop up to distrupt the airflow (or spoil the airflow) that passes over the top of the wing. This kills the lift for that part of the wing and causes the aircraft to descend. The spoilers are deployed by inputs from the pilot when he wants to descend. During landing, the aircraft computer detects when the aircraft touches down on the runway and automatically deploys them to ensure the aircraft doesn't bounce up and try to keep fllying and helps slow it down quicker.