Bernoulli's principle states that - P + ρgh + 1/2*ρ*v2 = constant for a fluid.
Now when the plane is in the air, the wings of the plane are designed such that it takes more time for the air molecules towards the bottom surface of the wings to cross the wings as compare to the upper surface. This results in lower velocity of fluid i.e. air towards the bottom of the wing. Now applying the principle, we get higher pressure for the fluid in the bottom (so as to conserve the overall formula). Thus pressure at the bottom is higher than pressure at the top so there is a net upward force on the wings which provide the lift.