Refer to figure.
FLUTTER AND RESONANCE
Flutter is an uncontrolled oscillation that can occur on fixed surfaces, such as the wing or on control surfaces such as the ailerons or elevators.

Flutter is caused by the interaction of aerodynamic forces, inertia forces and the elastic properties of the surface or structure and can lead to the catastrophic failure of the structure.

An aircraft that is in the cruise will be supported by its wings and they will be bent upwards. If the aircraft is subjected to a gust it will shake up and down with the wings flapping at a certain frequency. If the vibration is similar to that of the structure of the wing then it will begin to resonate. The resonance will amplify the flutter and may well lead to failure of the structure.

Flutter must not occur within the normal flight operating envelope of the aircraft. Flutter can be prevented by mass balancing control surfaces to alter the moment of inertia of the surface and therefore the period of vibration (move the control surface C of G from behind the hinge to closer to the hinge).

Poorly maintained aircraft, particularly those with excessive control surface backlash (play) or flexibility may mean that flutter could occur at speeds below the limit airspeed.