The molecules in the air are constantly moving around and bumping into each other, or other objects, in this case the wings. When air molecules are trapped inside a container such as a bag, the net force they exert on the walls of the container due to their motion is known as pressure. The energy due to movement is called kinetic energy. In other words, pressure is a measure of the kinetic energy.
Therefore if the pressure is higher below the wing, the kinetic energy of that air is higher than that of the air above the wing. The air molecules are either hitting the bottom of the wing faster and thus harder, or there are simply more air molecules smashing against the bottom of the wing.