The net force on an airplane in level flight with constant velocity is zero. Gravity is exerting a downward force on it, which is the airplane's weight. There must be an upward force acting on the airplane. This upward force is due to the interaction of the airplane with the surrounding air, which is, in the airplane's reference frame, rushing past the airplane. This upward force is called lift. We know that this moving air also exerts a force called drag on the airplane, opposing the relative motion of plane and air. To cancel the drag, the airplane must be acted on by a force in the direction of its velocity relative to the air. This force is called thrust.
For an airplane in straight and level flight, thrust and drag point along the horizontal and lift and weight point along the vertical direction.