Refer to figure.
Spoilers are panels on the upper wing surface that are movable. They are hinged at the forward edge and are used to disturb the airflow over the wing to reduce lift. They will also increase drag on the wing when raised.

When turning, the downward-going wing will experience a decrease in lift with an upward-going aileron. This decrease in lift can be assisted by raising the spoiler as well. Raising the spoiler on the downward-going wing also increases drag on that wing, which helps to counter adverse yaw. The degree of spoiler that needs to be raised varies proportionally to the aileron input, allowing for versatility in adding drag and decreasing lift. At the same time, the spoiler on the upward-going wing will remain flush.