Electrical Insulator
Electrical Insulator is a material that blocks electric charge from flowing through it.
Think of it this way
A rubber oven mitt sits between your hand and a hot pan. The rubber blocks heat the same way an insulator blocks electric charge. Charge tries to move, but the rubber stops it cold. The mitt keeps the flow on one side and away from the other.
Explaining electrical insulator by grade level
Some things let charges move through them, and some things stop charges cold. When you rub a foam plate with a wool sock, the charges stay on the foam. They cannot pass through it. The foam acts like a wall that traps the charge in place.