Pulley
Pulley is a wheel with a rope around it that makes heavy things easier to lift.
Think of it this way
A flag on a flagpole uses a pulley at the top. You pull the rope down, and the flag goes up. The rope loops over a wheel at the top, changing the direction of your pull. One end goes down as the other end goes up.
Explaining pulley by grade level
Think about a model elevator. A rope goes over a small wheel at the top. When you pull the rope down, the elevator box goes up. The wheel changes which way you pull so lifting feels easier.