Counterweight
A counterweight is a heavy object that balances a load so it moves up and down with less effort.
Think of it this way
A seesaw made of a plank balanced on a block shows this well. Put a heavy jar on one end and a lighter jar on the other. The heavy jar pushes its end down and lifts the light jar up. Add weight to the light end and the two jars balance — neither side takes much effort to move.
Explaining counterweight by grade level
Think of a model elevator. One side holds the box that carries people. The other side holds a heavy weight. That weight helps pull the box up with less work.