“The wing of an airplane is curved from the front to the back. The curve on the top half of the wing creates low pressure above the wing which acts like a vacuum cleaner, “sucking” the wing upwards along with everything attached to it –– like the body of the airplane. This “sucking” force is called lift.
In general, when the amount of lift is exactly equal to the weight of the aircraft, the airplane will fly in level flight, unable to climb up or down. If the amount of lift exceeds the weight, the plane will climb higher. If the amount of lift is less than the weight of the airplane, the plane will descend.
A pilot can change the amount of the suction or lift by either slightly raising the nose of the airplane or by going faster and increasing the power to the engine.”
So there you have it. Weight and lift and moving forward. The simple explanation behind a flying metal bird.