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