Cyanohydrin formation:

This reaction is nucleophilic addition reaction.
Step (i): The nucleophilic carbon in the cyanide adds to the electrophilic carbon in the polar carbonyl group, electrons from the $\text{C}\text{=}\text{O}$ move to the electronegative $\text{O}$ creating an intermediate alkoxide.
Step (ii): An $\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}/\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}$ reaction protonation of the alkoxide oxygen creates the cyanohydrin product.