During flight, the four forces acting on the airplane are LIFT, WEIGHT, THRUST, and DRAG.
LIFT is the upward force created by the effect of airflow as it passes over and under the wing.
The airplane is supported in flight by lift.
WEIGHT is the opposing force to LIFT, it is caused by the downward pull of gravity.
THRUST is the forward force that propels the airplane through the air.
THRUST varies with amount of engine power being used by the pilot.
The opposing force to thrust is DRAG, this force is a backward or retarding force which limits the speed of the airplane.
In unaccelerated flight, the four forces are in EQUILIBRIUM.