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][1]: ![Airfoil][2] [1]: http://www.daviddarling.info/images/airfoil_diagram.jpg [2]: https://i.sstatic.net/tYVO9.jpg