The atoms of a solid have electrons rotating in orbits around the positive nucleus. This is true of gases and liquids as well. These orbiting electrons exist in energy shells or levels.
The potential energy (energy of position) increases with distance out from the nucleus. The outermost occupied energy level is called the valence shell. This is a higher energy level than the energy levels of electrons in the other shells since the electrons are rotating further from the nucleus.
The electrons in the valence shell can most easily pass from one atom to another and thus constitute an electric current. Furthermore, the valence electrons are the ones that go into chemical reactions, or combinations, with other atoms.
When an outside influence such as an electric field or heat is applied, a valence electron may acquire sufficient energy to jump through a forbidden (energy) gap and on into the conductor band where it is free of any influence of the positive nucleus and becomes a carrier of electricity, ready to take the place of another electron that has just left its own atom, in the same manner.