electron delocalization
Imagine money kept in one person's pocket versus money pooled into a shared account that the whole group can draw on. Electrons can be like either. Most are 'localized', tied to one atom or one bond; but some are pooled across several atoms, free to roam over a wider region. That spreading-out is electron delocalization.
Precisely, electron delocalization is the spreading of bonding electrons over three or more atoms rather than being confined to a single bond between two atoms. It happens when atomic orbitals overlap continuously across a chain or ring, letting electrons occupy molecular orbitals that span the whole system. It is the honest, modern picture of what the older language of resonance was trying to capture.
Delocalization matters because spreading electrons out lowers their energy and makes molecules extra stable — this 'delocalization stabilization' is why benzene resists reactions that would break its ring. It also enables electrical conduction: in metals and graphite, delocalized electrons roam freely and carry current. The caveat is that delocalization is a continuum, not a yes/no switch; bonds can be a little or a lot delocalized.
In benzene (C₆H₆), six electrons are not stuck in three separate double bonds. They are delocalized in a ring above and below the carbon plane, so all six C–C bonds are identical and intermediate between single and double. This shared cloud makes benzene unusually stable and far less reactive than its drawing suggests.
Benzene's ring of delocalized electrons makes all six bonds equal and stable.
Resonance and delocalization describe the same reality with different tools. Resonance is the old Lewis-structure workaround — drawing several forms; delocalization is the direct molecular-orbital description — electrons genuinely spread over many atoms. The molecule is not flipping between forms; the electrons are simply not where a single line-drawing puts them.