Four forces are at work in flight: weight (a downward force caused by gravity), lift (an upward force caused by air moving over the wings), drag (a backward force caused by air resistance), and thrust (a forward force caused by an engine).
For a plane to fly, there are two requirements. First, the force of lift must exceed that of weight, so that the plane goes up instead of down. All air exerts pressure, but the faster it moves, the less it exerts. When a wing cuts through the air, it disturbs the air stream. Because the wing’s surface is curved, the air traveling over it has farther to go than the air below it and so must move faster. As a result, the air above the wing exerts less pressure than the air below it. The air below the wing pushes up more air than the air above it, resulting in the upward force of lift.
Additionally, the force of thrust must overcome that of drag. This is accomplished by the thrust generated by the engine. The engine exerts enough thrust to overcome drag, so the airplane goes forward and not back.