In order for electrons (and thus electricity) to flow, energy must be provided to the electrons so that that they can break free from their orbits. If you rub a balloon, you are providing the energy. In a nuclear power station, nuclear fusion is providing the energy.
Click the cloud with your mouse and watch what happens to the electrons when provided with energy.
The atomic structure of metals is such that the electrons in the outer shell are not held strongly. This enables the electrons to wonder from atom to atom when provided with energy. In an insulator the electrons are held more strongly thus conduction of electricity (flow of electrons) is much more difficult.
