Brine is a solution of sodium chloride and water. The electrolysis of brine is a large-scale process used to manufacture chlorine from salt.

 At anode, chloride $\left({\mathrm{Cl}}^{-}\right)$ ions are oxidised by electron loss to form chlorine gas. This is an oxidation reaction because the chloride ions lose electrons.

${\mathrm{Cl}}_{}^{-} \left(\mathrm{aq}\right)\to \frac{1}{2}{\mathrm{Cl}}_2 ( \mathrm{g})+{\mathrm{e}}^{-}$

The net process is the electrolysis of an aqueous solution of $\mathrm{NaCl}$ into industrially useful products sodium hydroxide $\left(\mathrm{NaOH}\right)$ and chlorine gas.