It also refers to an airplane's response to the pilot's use of the controls. The CG is an imaginary point about which the weight of an airplane balances. If you put a ruler across your finger and place it so it balances, your finger is at the CG of the ruler. The CG of an airplane does not stay at the same place at all times. The loading of heavy cargo onto an airplane will shift the CG as will the drainage of fuel from the tanks during flight.
Pilots have to recognize shifts in the CG and respond accordingly.
Sudden shifts in the CG can be catastrophic. For example, if an airplane experiences turbulence during flight and a large cargo load shifts, the pilot may have trouble reacting quickly enough to the change in the airplane's center of gravity to maintain stability of the airplane. Imagine putting a weight on the back of an arrow. Because the CG has shifted, it will be less stable and tend to wobble. Now consider the arrow flying with feathers in front.