The energy bands are allowed values of energy for the electrons. In the picture the energy levels of carbon atoms are shown. If you have a large interatomic distance between atoms (the right of the picture) the atoms won't interact much. The energies are spaced far apart and many atoms can share the same energy level because they are not in danger of violating the Pauli exclusion principle.
If the atoms get closer together the exclusion principle comes into play and the energy levels split into many lines. So many that it looks like a band. In this band the electrons can hop to unoccupied levels because they so are close together. Thermal energy is sufficient to change between levels.
If the atoms get even closer together the band splits again into a conductance band and valence band. This time the electrons can still move inside the bands but the gap is so large that the chance of changing between valence and conduction is very slim.
Metals fall into this middle regime where the 'conduction band and valence band overlap'. You can't actually tell these apart in this regime so when you say the valence band and conduction band overlap you just mean that they are joined.
Note: to predict the actual conductivity of a material you also need to consider how the energy levels are filled.
Image source: https://en.wikipedia.org/wiki/Electronic_band_structure#/media/File:Solid_state_electronic_band_structure.svg