A Dielectric is any substance that acts as an insulator against electricity. It is often important to electrically isolate structures from each other.
Using sacrificial anodes to protect steel fuel tanks is common. It is necessary that electric current does not flow from the tank to any steel pipe connected to the tank.
Electrically Isolating the two structures is important. Bushings are fittings made of a dielectric substance such as nylon that is not a conductor of electricity. They isolate the structures when used.