* Insulators resist the flow of electricity. They have tightly bound electrons that don't move freely.

* Conductors allow electricity to flow easily through them. They have loosely bound electrons that can move freely.

Rubber is a good insulator because its electrons are tightly bound. This makes it difficult for electricity to pass through it.

Think of it this way: If you try to push a bunch of marbles through a tightly packed container, they won't move easily. That's like electricity trying to flow through an insulator.