Refer to figure.
CENTER OF GRAVITY (C.G.)

The total mass of the aircraft can be said to act through one point, called the Centre of Gravity (C.G.). To ensure the aircraft is safe to fly, the center of gravity must fall within specified limits established by the aircraft manufacturer (between the front and the rear C.G. limit). The C.G. has to be calculated before each flight to make sure the aircraft within the safe envelope, taking into account the weights and the moments of the loads.

Proper C.G. location is crucial for maintaining an aircraft's stability and controllability. Loads that are not adequately secured might shift during flight, altering the position of the C.G. This sudden movement of the C.G. can make the aircraft difficult to control, and there is a possibility that the new C.G. location might be outside of the allowable limits. Ensuring that loads are properly secured prevents such shifts and maintains safe flight conditions.