Bernoulli's Principle allows us defy the affects of gravity in a fixed wing aircraft. Bernoulli's Principle states that as a fluid (in this case gas) increases in velocity, the pressure decreases. In other words, this is what causes out ears to pop when we roll up the windows in a fast moving car, and windows to be blown out of a high rise downtown building.
An airplane is able to fly because the top part of the wings are curved. As air is pushed over the top of the rounded wing, the air velocity increases and is moving faster than the air flowing past the bottom part of the wing. The split in air velocities causes the pressure on the top of the wing to decrease. This means that the air rushing across the bottom part of the wing to be greater than the top and the pressure pushes the plane up. This is called lift.