Electrons orbit the positive nucleus of an individual atom in permitted energy levels, as shown by the grey lines on the left of the diagram below.
In a large collection of atoms, eg a metal wire or a semiconductor crystal, the energy levels become reorganised into two bands. This is shown on the right of the diagram:
Electrons can’t exist in the energy 'gap' between bands.
Conduction is a movement of electrons in a solid. For conduction to occur there must be: