3 Na2S2O3 (aq) + 8 KMnO4 (aq) + H2O (l) → 3 Na2SO4 (aq) + 8 MnO2 (s) + 3 K2SO4 (aq) + 2 KOH (aq)
This is an oxidation-reduction (redox) reaction:
8 MnVII + 24 e- → 8 MnIV (reduction)
6 SII - 24 e- → 6 SVI (oxidation)
KMnO4 is an oxidizing agent, Na2S2O3 is a reducing agent.