5 NaNO2 (aq) + 2 KMnO4 (aq) + H2O (l) → 2 MnO (s) + 5 NaNO3 (aq) + 2 KOH (aq)
This is an oxidation-reduction (redox) reaction:
2 MnVII + 10 e- → 2 MnII (reduction)
5 NIII - 10 e- → 5 NV (oxidation)
KMnO4 is an oxidizing agent, NaNO2 is a reducing agent.