Arenes and the benzene ring
Arenes are hydrocarbons that contain one or more aromatic rings. The simplest arene is benzene, C6H6. Its six carbon atoms form a planar regular hexagon. Each carbon is sp2-hybridized, makes three sigma bonds, and contributes one electron to a delocalized pi system above and below the ring.
The two Kekulé drawings with alternating double bonds are resonance contributors, not two substances rapidly changing into each other. The real molecule has six equal C–C bonds with lengths and energies between typical single and double bonds. A circle inside a hexagon is a useful shorthand for the delocalized six-electron pi system. Delocalization lowers the energy of the ring and is called aromatic stabilization.