Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force is equal to the natural frequency of the system on which it acts.
If the molecule has several resonance structures then the molecule is considered to be more stable.
When a molecule has a greater number of resonance structures then it is found to be more stable compared to the molecules with a lesser number of resonance structures.
Resonance allows for the delocalization process, and the overall energy of the molecule gets reduced.