You need to push backward when you want to move forward. When you're walking down the street, your feet push back against the sidewalk to move you forward. In a car, the wheels do something similar by kicking back against the road. But what about the propellers that power airplanes?
A propeller is a machine that moves you forward through a fluid (a liquid or gas) when you turn it. It has several twisted blades poking out at angles from a central hub, spun around by an engine or motor. Propeller produces lift in a forward direction, a force we refer to as thrust. Its rotary motion through the air creates a difference in air pressure between the front and back surfaces of its blades. This pressure difference pushes the plane forward.