The band gap is the energy difference between the valence band and the
conduction band. The dark shading represents electrons in the band. Insulators
have a large band gap, and metals have a small band gap. Semiconductors
are the intermediate case.
Applying a voltage to a metal tips the bands, resulting in a flow of electrons.
When electrons from the conduction band recombine with holes from the valence
band, the energy released is in the form of light.