Lewis structures are often inadequate, without the concept of resonance, to describe the distribution of electrons in certain molecules and ions.
Sometimes, more than one Lewis structure is needed to describe a molecule. These structures differ only in the placement of electrons. Resonance describes more fully the distribution of electrons in a molecule.
A resonance hybrid is the true structure of a molecule or polyatomic ion, whereas the various resonance structures that are used to depict the hybrid do not individually have any reality.
The hybrid is a mix, or average, of the various resonance structures that compose it.
