What do you understand by resonance?

There are many covalent compounds in which a single definite structure of the compound is not capable of explaining all of its properties. To explain all the properties of such a compound we need the help of two or more structures and the actual compound is somewhat intermediate of these two or more structures i.e. actual compound is the hybrid of these two or more different structures. Thus resonance may be defined as a phenomenon in which a single compound is supposed to be existing as a hybrid of two or more structures differing in the distribution of electrons and not of atoms. These different structures of a molecule are known as contributing structures or resonating structures or canonical forms. No one of the contributing structure truly represents the molecule, but each one of them contributes to the final i.e. actual molecule is intermediate between all the contributing structures. Different contributing structures are written by putting a double head arrow (↔) between them. For example,
(i) Benzene molecule is a resonance hybrid of the following two main contributing structures:

(ii) SO₂ is a hybrid of the following two contributing structures: