4 Cl2 (g) + 10 NaOH (aq) + Na2S2O3 (aq) → 5 H2O (l) + 8 NaCl (aq) + 2 Na2SO4 (aq)
This is an oxidation-reduction (redox) reaction:
2 SII - 8 e- → 2 SVI (oxidation)
8 Cl0 + 8 e- → 8 Cl-I (reduction)
Na2S2O3 is a reducing agent, Cl2 is an oxidizing agent.