Refer to figures.
The point where an aircraft’s total sum of a pressure field is acting upon the body is called the Center of Pressure (CP).
This is the point where the resultant vector of lift is said to be operating. As the airfoil or Angle of attack changes (thus changing the lift coefficient),
this pressure field will change and so will the center of pressure.

Aircraft longitudinal stability is directly related to Centre of Gravity (CG) position.
An aircraft with a more forward CG is more longitudinally stable than an aircraft with a more aft CG. Placing the CG beyond the forward limit can cause an unacceptably high value of longitudinal stability, decreasing manoeuvrability by a significant amount.
Place the CG beyond the aft limit and the aircraft is too unstable, having excessive manoeuvrability.

If an aircraft is longitudinally stable, an increase in Angle of Attack (α) results in a negative pitching moment.
The nose will slightly fall down, decreasing the Angle of Attack. If this aircraft has a decrease of Angle of Attack, the nose will come slightly upwards again,
creating a positive pitching moment again by slightly raising the nose.

If an aircraft is very stable (forward CG), the elevator will have to produce a bigger moment to move the pitch and thus requires a larger deflection, as the forces on the tailplane often are deflected downwards.