BaS (aq) + 4 NaHSO3 (aq) → 2 Na2S2O3 (aq) + BaSO4 (s) + 2 H2O (l)
This is an oxidation-reduction (redox) reaction:
S-II - 4 e- → SII (oxidation)
S-II - 8 e- → SVI (oxidation)
4 SIV + 8 e- → 4 SII (reduction)
SIV - 2 e- → SVI (oxidation)
BaS is a reducing agent, BaS is a reducing agent, NaHSO3 is an oxidizing agent, NaHSO3 is a reducing agent.
This is a precipitation reaction: BaSO4 is the formed precipitate.