Band theory of solids expresses the quantum state that an electron takes inside a metal solid. Every molecule comprises various discrete energy levels. In the band theory, you will get to know how the electrons behave inside a molecule.
The energy band that consists of the valence electrons' energy levels is called the valence band. The valence band is available just below the conduction band and the electrons of this band are loosely bound to the nucleus of the atom.
The energy band comprises free electrons energy levels, known as the conduction band. For electrons to be free, external energy must be applied such that the valence electrons get struck to the conduction band and become free.
The energy gap between the valence band and the conduction band is commonly known as the forbidden band which is also known as the forbidden gap. The electrical conductivity of a solid is set by the forbidden gap and the classification of the materials is done as conductors, semiconductors, and insulators.