When an airplane's flight path is straight and level, it is in a balanced state. Since there is no change on the forces that work on it, nothing will change in the position, speed and direction of the airplane.
A gust of wind can change the attitude of the airplane during flight. If this movement creates an opposite force - the airplane will return to its original position - this is called a stable flight.
If this movement creates a force that enhances the change - the airplane will increase the movement - this is called an unstable flight.
If this movement does not create any new force - the airplane will stay in the new position - this is called neutral stability.