No information is given about
What are canonical structures ?
What are resonance hybrid ?
Many molecules do not exist in one structure but they exist in many structures. These different structures are called canonical structures or resonating structures. Thus a molecule can not be represented by any one of the canonical structures. Hybrid of all the canonical structures is known as hybrid structure that can be supposed to be true structure. Always the hybrid structure has lesser energy than the most stable canonical form.
Resonance energy is the difference between the actual bond energy of the molecule and that of the most stable of the resonating structures.
Resonance energy = (Actual bond energy) – (Energy of most stable canonical form)
These resonating structures are only hypothetical concept to understand the molecule in better way. They do not exist for even a fraction of time. But for study purpose we assume such things.