A fluid that flows past a surface of a body exerts a force on it. Lift is the perpendicular component to the oncoming flow, of this force. It is the opposite of a drag force.
Aerodynamic lift is different from other kinds of lift. Aerodynamic lift requires relative motion of the fluid. It usually refers to when an object is completely immersed in a fluid.
Lift is what makes it possible for a plane to fly. Lift is also used in Frisbees. As man takes to the sky, we owe much to the power of lift forces.