[Physics] Pressure difference between 2 side of wings of aircraft

Question

I understand why the pressure is reduced above the wings of aircraft, but when I read books and look for information, it says that the air is compressed below the wings, I do not understand why it is? Could you tell me what makes the air compressed below the wing, and how is it compressed?

Answer 1

The simple answer is that a wing moves through the air generally at a non-zero angle of attack. The air flow below the wing sort of impacts the wing surface, compressing and slowing down as it is deflected.

See this Drawing: