Electricity! Sometimes it flows in wires to your toaster or your TV. It makes the lamp on your desk glow bright when you flip the switch. Why does the electricity stay in the wire and not jump out?
All matter has tiny amounts of electricity. In some materials this electricity can move quickly. These materials are said to be good conductors.
Other materials don't swap electric charges very readily. Since the electric charges don't move, electricity doesn't flow through the material. These materials are said to be good insulators.