Lift and the Center of Pressure Diagram

Lift is the vertical (up and down) force acting on a wing. The focal point of this lift force is called the center of pressure. In flight, as the wing changes its angle to the oncoming flow of air, the center of pressure moves back and forth along the surface of the wing. 

When the center of pressure and the center of gravity coincide, the aircraft continues in level flight. If the center of pressure moves behind the center of gravity, the airplane pitches down. If it moves ahead of the center of gravity, the airplane pitches up. To control climb and descent, the pilot must be able to control the movement of the center of pressure. 

In this diagram, the green dot represents when the center of gravity and center of pressure coincide, or are in the same position. The yellow dot, which has an arrow pointing to it, represents the center of pressure. The blue dot represents the center of gravity.