4 S (s) + 6 NaOH (aq) → Na2S2O3 (aq) + 2 Na2S (aq) + 3 H2O (l)
This is an oxidation-reduction (redox) reaction:
2 S0 - 4 e- → 2 SII (oxidation)
2 S0 + 4 e- → 2 S-II (reduction)
S is both an oxidizing and a reducing agent (disproportionation (dismutation)).