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Propellers produce thrust to make aircraft fly. Propellers use a difference of air pressure to help thrust aircraft forward or up. This relates to Bernoulli's Principle. Bernoulli's Principle states that faster moving air has lower air pressure and slower moving air has higher air pressure. Air pressure is like a push by air. When a propeller begins to turn, the air on top of the propeller (if it is facing up and down) moves faster than the air on bottom.  This means that the air on bottom has a higher push that pushes the air craft up (or forward)! Not all airplanes use propellers however. Many commercial airplanes use jet engine thrust instead. Helicopters are the most common example of aircraft that still rely on propeller thrust.