Detailed description of hydroxy(phenyl)methanesulfonic acid?
Hydroxy(phenyl)methanesulfonate is a compound of a class commonly called "bisulfite addition products" or "bisulfite adducts", since bisulfites are commonly used for the reaction you have mentioned. But there's an equilibrium in aqueous environment between metabisulfites and bisulfites
$$\ce{S2O5^{2-} + H2O \rightleftharpoons 2HSO3^{-} }$$
in favor of the bisulfite, this is why metabisulfites can be used as well, probably.

The bisulfite anion is proposed to have two tautomers:



The form at the left has a lone electron pair at the sulfur atom, and can act as a nucleophile.

The carbonyl/aldehyde group $\ce{&gt;C=O}$ has a partial positive charge at the carbon atom.

The reaction in now no mystery, it's a nucleophilic addition to the carbonyl group.

If I may cite a crude reaction mechanism from Clayden/Greeves/Warren's Organic Chemistry, short chapter Bisulfite addition compounds:


And from the March's Advanced Organic Chemistry:


  The reaction is reversible (by treatment of the addition product
  with either acid or base) and is useful for the purification of the starting compounds, since the addition products are soluble in water
  and many of the impurities are not.


If you want more details (kinetics, etc.), there is freely available couple of scientific articles, Sulphur isotope effects in the bisulphite addition reaction of aldehydes and ketones


I. Equilibrium effect and the structure of the addition product
II. Bond-formation effect