The trailing edge of a large aircraft has lots of panels which move in/out/up/down.
Starting from the aircraft body, you will typically find firstly a section of wing flap. During cruise, it is retracted into the wing. During takeoff and landing, it comes out and down. This makes the wing generate lift at slower speeds, and also adds drag. It comes out in stages, selectable by the pilot. The slower the aircraft needs to fly, the more the flap comes out. By the way, at the front, or leading edge of the wing are slats. These also make the wing generate lift at slower speeds.
Next is typically the highspeed aileron. This moves up and down. If it moves up, the wing generates slightly less lift, and the aircraft will bank towards the wing. If it moves down, the wing generates slightly more lift and the aircraft will bank away from the
wing. This is called the high-speed aileron because it is used during both lowspeed and highspeed flight.
Next comes more flaps. This is usually the largest section of flaps on the wing.
Finally, you get to the low-speed aileron. This does the same as the high-speed aileron, but is only used at lowish (less than 200mph-ish, depends on the aircraft). speeds. It behaves like the high-speed aileron. This will be the bit you saw apparently bouncing about in the breeze. Like all the other bits of wing, it is under control of the pilot (or autopilot), and does at times move fairly abruptly up and down. Its supposed to!
The ailerons on each wing work together, so when the ailerons on the left wing go down, the ailerons on the right wing go up and the aircraft will bank to the right.
On top of the wings are spoilers (or airbrakes). These have several purposes. At low speeds, they help bank the aircraft. You can sometimes see them moving up slightly when the lowspeed aileron moves up.
The spoilers are also used to slow the aircraft down and let it descend quickly without building up forward speed, for example when air traffic control tell the pilots they have to descend to a certain altitude without exceeding a certain speed. You can sometimes feel and hear a gentle rumble when they are up. Finally, the spilers are used on touchdown to both slow the aircraft down and to significantly reduce the lift the wing is producing, so that more weight is on the wheels.
These descriptions are generic, and do vary a lot among aircraft. For example, some large jets have ailerons that droops down during takeoff and landing, so they also act as flaps!
Fascinating things, aircraft wings.