We are looking at the relative strengths of H2S versus H2SO3. Both are acids and in water will ionize into a proton and the conjugate base. The equations for that are below.
H2SO3 <--> H+ + HSO3-
H2S <--> H+ + HS-
The equations above are called acid dissociation equations. The equilibrium constant is a way to measure what percentage of each acid is in the dissociated state (products) versus the associated state (reactant). The Ka for H2SO3 (sulfurous acid) is 0.016 and for H2S (hydrogen sulfide) it is 6.3x10^-8. The smaller the Ka, the weaker the acid. Since H2SO3 has the higher Ka value, it is the stronger acid of the two.