H2O (l) + 2 KMnO4 (aq) + 3 NaNO2 (aq) → 2 KOH (aq) + 2 MnO2 (s) + 3 NaNO3 (aq)
This is an oxidation-reduction (redox) reaction:
2 MnVII + 6 e- → 2 MnIV (reduction)
3 NIII - 6 e- → 3 NV (oxidation)
KMnO4 is an oxidizing agent, NaNO2 is a reducing agent.