Bernoulli's Principle

Applied to Aircrafts

When an airflow moves through a restricted pipe, the speed of the airflow increases and the pressure decreases.

If you look at the top of an airfoil, you have that restricted pipe, which means you have a decreased pressure, which means, the top of the airfoil has a lower pressure than under the airfoil (which isn't in a restricted pipe), therefore, lift is generated by the higher pressure trying to compensate for the lower pressure.