There are many different ways in which we could accurately describe the lift forces acting on the aerofoil.
METHOD 1.
One way is to add together all of the individual lift forces to produce a single lift force. This single lift forces acts at the centre of pressure.
Because it acts at the centre of pressure it does not exert any pitching moments about the centre of pressure.
So we have lift expressed as a single force, (but no pitching moment) acting at the centre of pressure.
But if we were to measure the moments about any other point on the aerofoil we would find that the lift exerts a moments that is the product of the lift force multiplied by its distance from the centre of pressure.
This means that a second method of expressing the effect of lift is a force plus a pitching moment, both acting at a point that is not the centre of pressure.
METHOD 2
As LM’s diagram shows, if we measure the moments about the trailing edge we will find that increasing angle of attack produces an increasing pitch up moment.
If we take moments about a point slightly ahead of the trailing edge we will find that the rate of increase of pitch up moments is less than it was at the trailing edge.
If we gradually move forward from the trailing edge the rate of increase of pitch up moments will gradually decrease.
If we take moments about the leading edge we will find that increasing angle of attack increases the nose down moment.
If we take moments about a point slightly behind the leading edge we will find that the rate of increase of pitch down moments is less than it was at the leading edge.
If we gradually move aft from the leading edge the rate of increase of pitch down moments will gradually decrease.
If we repeat this process, gradually moving forward from the trailing edge and aft from the leading edge we will find a point at which the pitching moments do not change with changes in angle of attack. This point is the Aerodynamic Centre of the aerofoil.
So we can express the effects of lift as a single force plus a constant pitching moment acting at the Aerodynamic Centre.