Imagine a theatre with several levels, you can swap seats on the same level fairly easily, but moving to another level requires considerable effort, this is like the band gap in an electron shell system.the whole idea of charge is very interesting, when you charge a cap, where do the electrons go? you can not force them into the steel atoms, right? nor can you strip electrons off steel atoms to create negative charge? kind of like holes and electrons in transistor theory, borders on the conceptual side,
In conductors like metals, the electron band gaps overlap with the valency band and electrical conduction is possible. A large band gap in a material makes it an insulator. If a very high voltage would be necessary to cause an electron flow across the band gap, it would be an excellent insulator.
In a capacitor, the "plates" are separated by an insulating material which can only store charge as static electricity on its surface. It must be part of a very rigid mechanical system to counter the electro-mechanical forces generated between the opposing plates/foils due to their attraction: they want to touch to cancel the charge. How well this is accomplished defines the quality of a capacitor.
DaveP