Lift is the key aerodynamic force. It is the upward force created by the effect of airflow as it passes over and under the wings.
The basic principle of lift is derived from a scientific principle called Bernoulli's Principle. The basic result of this principle with regard to lift is that when a greater amount of pressure exists under the wing than over the wing, there exists a lifting action on the airplane. You have probably experimented with this principle yourself whenever you have stuck your hand out the window of a moving car. The force that pushes your hand up is called lift.
Lift is the force that opposes weight. In straight-and-level, unaccelerated flight, lift and weight are equal.