Rubber is a good insulator because it prevents electrons from moving through it easily and gaining energy, which causes an electrical charge. Electrons are transferred to rubber, but they cannot move fast enough to gain the energy they need to disperse an electrical charge.
Because rubber is a good insulator, it is used on the outside of wires that are used to conduct electricity. An insulator is a material that reduces or stops the flow of an electrical current. Rubber provides protection from the dangers of electrical charges being carried through the metal conductors.
In rubber, the electrons are tightly bound within the material, which means that they are not free to be shared by neighboring atoms. The properties of rubber allow the electrons to slow down. In some cases rubber can completely prevent electrons from moving.
Electrical cords are a good example of rubber being used as an insulator. If a cord is frayed and has damaged the outer rubber lining, the rubber will not be an effective insulator. If the conductors (wires) are exposed, it is necessary to replace the cord.
Keeping electricity bound within the material is the primary goal of an insulator, and because of this, rubber is a very good choice.