The airfoil's curve guarantees that the air flowing over the top surface travels faster than the air passing beneath the wing. Meanwhile, the air flowing beneath the wing is of relatively higher pressure. And it's this pressure differential that generates the airplane's lift. It works the same way with bird wings, but there's a big difference that we'll explain shortly. As a child, you've probably conducted a crude experiment that illustrates the principle of lift.
Weight acts against lift and works in a downward direction.
Thrust is created by the propeller or, in the case of a jet, the outflow of a turbine engine. Drag is thrust's enemy, so the pilot has to apply enough engine power to overcome it. Drag comes in many forms, and it can add up quickly. So it follows that the draggier the airplane, the more thrust will be required. In steady flight, lift, weight, thrust, and drag all balance each other.