The air that flows over the top of the wing moves faster than the air moves along the bottom. The air that is moving faster exerts less force than the slower moving air. Thus, the air pushes down on top of the wing less than the air pushes up on the bottom of the wing. The greater push upward is the lift of the wing. Lift holds an airplane up.
Lift acts in opposition to the weight of an airplane, which acts downward.
Weight is the force generated by gravity. Weight always acts downward toward the Earth. For an airplane to fly, lift must equal weight. Weight holds an airplane down. Thrust is the forward force that is produced by the pull of a propeller or the push jet exhaust.