I heard that when an airplane is flying, air speed or air pressure is higher under the wings. Is it true?
In case of a subsonic attached flow over the cambered airfoil (or a symmetric one at an angle of attack), the airflow speed is higher over the wing and pressure is lower. It is the other way around on the lower surface.
Note that this is just the mechanics of the flow over the wing. This is not an explanation of lift.