what do you mean by forbidden gap?(solid state chemistry)

When same atoms may be two or more atoms come close together in solid crystal lattice, the electrons present in outer orbital come under the influence of forces from other atoms which result in formation of large no of energy levels to come together and result in formation of two band. There are two energy bands are valence band(filled electron level) and the conduction band(empty levels) . Separation between these two bands is called as energy gap and this forbidden gap is that gap in which electrons cannot exist.