energy band diagram of isolated pn junction

To understand the concept you will have to go into deep quantum mechanics of the crystal.

If we bypass all the calculations then we can understand it as:

 

If we derieve the energy levels possible in a system, they come out to be discreet instead of continuous, as opposed to continuous levels in case of classical models.

Thus in a crystals, as we go higher in energy, the differencce between these levels decreases and they can be approximated as a band, called of band of energies.

In solids, the last filled energy band is called valance band.

and the band just above it is called conduction band. and the difference between these bands is called Energy Gap.