What is resonance enthalpy? What is its significance?

The actual enthalpy of the molecule (i.e. enthalpy of resonance hybrid) is always found to be less than the enthalpy of any of its contributing structures. This difference in enthalpy is called resonance enthalpy. It is defined as the difference in enthalpy between the enthalpy of the most stable of the contributing structures and the enthalpy of the actual molecule (resonance hybrid).
Resonance enthalpy = Enthalpy of the most stable contributing structure -  enthalpy of resonance hybrid.
Due to the low fo resonance hybrid, it is more stable than any of its contributing structures. In other words, it can be said that resonance hybrid is always more stable because of resonance enthalpy. Larger the resonance enthalpy, more will be the stability of the compound. Also, relative stabilities of two hybrids can be compared in terms of resonance enthalpies.