4 Na2SO3 + K2Cr2O7 + 3 H2SO4 → Cr2(SO4)2 + 4 Na2SO4 + K2SO4 + 3 H2O
This is an acid-base reaction (neutralization): Na2SO3 is a base, H2SO4 is an acid.
This is an oxidation-reduction (redox) reaction:
2 SIV + 4 e- → 2 SII (reduction)
2 SIV - 4 e- → 2 SVI (oxidation)
Na2SO3 is both an oxidizing and a reducing agent (disproportionation (dismutation)).