The force of lift is the upward force that keeps an aircraft in the air. It's an aerodynamic force that's generated when a fluid, like air, flows past a solid object, like an airplane wing:
While the common meaning of the term "lift" suggests an "upwards" action, lift can be in any direction. For instance, for a sail the lift is horizontal, and for the wing on a race car the lift is down.
There are a number of ways of explaining the production of lift; some are more complicated than others, some have been shown to be false. The simplest explanation is that the wing deflects air downward, and the reaction pushes the wing up.