Mechanical Advantage
Mechanical advantage is how much a simple machine multiplies your effort force to move or lift something.
Think of it this way
A jar lid is hard to open with your fingers alone. A long spoon tucked under the edge gives you more force on the lid. The longer the spoon handle, the less push you need to pop the lid off. That is mechanical advantage at work.
Explaining mechanical advantage by grade level
Think about pushing a heavy box up a ramp. You push longer, but it feels easier. The ramp helps you use less force. That is mechanical advantage. The ramp trades a longer push for a lighter push. You do the same work either way.