This slide shows what happens when the pilot deflects the spoilers on both wings.
Spoilers are small, hinged plates on the top portion of wings.
When the pilot activates the spoilers, the plates flip up into the air stream. The flow over the wing is disturbed by the spoiler, the drag of the wing is increased, and the lift is decreased. When the airplane lands on the runway, the pilot usually brings up the spoilers to kill the lift, keep the plane on the ground, and make the brakes work more efficiently. The lower the lift, the better the brakes work. The additional drag of the spoilers also slows the plane down.