Some of the compounds can be represented by two or more than two structures called resonating structure. This phenomenon of representing the molecules by more than one structure is called resonance.
Resonating structure differs from each other only in the arrangement of electrons.
The actual structure of the compounds cannot be represented by any of the resonating structures. Actual structure of the compound is hybrid of all the resonating structure called hybrid structure. In the figure below, first two are the resonating structures and third is the resonating hybrid of benzene.
The energy of hybrid structure is lower than any of the resonating structure. This dfference in energy between any of the resonating structure and hybrid structure is known as resonance energy.