https://www.wikiwand.com/en/Resonance_(chemistry)
A resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure. A molecule or ion with such delocalized electrons is represented by several contributing structures[2] (also called resonance structures or canonical structures).
These are found, for instance, in Aromatic compounds, like Benzene
Resonance is most often found in Conjugated systems