In benzene, the p orbitals of the carbon atoms above and below the plane of the molecule overlap sideways with each other. This means that 6 pi electrons are found in a delocalised electron cloud above and below the plane of the molecule. This is represented by a ring in the displayed formula.
However, because the electrons are spread out across the molecule, the overall electron density is less than that of a conventional double bond.
Oxidising agents, like KMnO4 reacts with localized pi bonds as in alkenes and alkynes, in benzene there are delocalized pi bonds (pi electronic cloud) so KMnO4 can not react.
The nitrating mixture is H2SO4 + HNO3