The Centre of Pressure is the average location of all of the pressure acting upon a body moving through a fluid.
As an aircraft moves through the atmosphere, the velocity of the air varies around the surfaces of the aircraft. As an example, the air is accelerated as it passes over the cambered surfaces of the aerofoils. This variation of air velocity, especially over the wing and tail surfaces, produces a variation in the local pressure at various places on the aircraft. The average location of the pressure variation is referred to as the centre of pressure. The total aerodynamic force can be considered to act through the centre of pressure and can be resolved into its two components, lift and drag.
The centre of pressure does not remain in a constant location. As the angle of attack changes, the local pressure at every point on the aerofoil also changes. This, in turn, causes a change in the location of the center of pressure.