Propeller
Propeller is a set of blades that spin to push air or water and move things forward.
Think of it this way
A hand mixer held over a bowl has flat metal blades that spin fast. The spinning blades push the batter outward and down in a circle. This same push-and-move action is how a propeller works. The blades catch air or water and shove it back, which pushes the whole object forward.
Explaining propeller by grade level
A propeller has blades that spin very fast. When the blades spin, they push air behind them. That push moves a plane or boat forward. Bigger blades can push more air at once, so the push is stronger.