Benzene exhibits resonance, a phenomenon where the actual electronic structure is a hybrid of two or more contributing structures.
In the case of benzene, there are two main resonance structures:
In one structure, double bonds are between C1-C2, C3-C4, and C5-C6.
In the other structure, double bonds are between C2-C3, C4-C5, and C6-C1.
These resonance structures imply that the π electrons are delocalized over all six carbon atoms.
The real structure of benzene is a resonance hybrid of these two structures, with equal bond lengths between all carbon atoms.
This delocalization of π electrons gives benzene extra stability, known as resonance energy.