What is meant by mechanism of a chemical reaction?

Draw the Fischer projection formulae of two products obtained when compound ($\mathrm{A}$) reacts with ${\mathrm{OH}}^{-}$ by ${\mathrm{S}}_{\mathrm{N}}1$ mechanism.

Draw the Fischer projection formulae of two product formed when compound ($\mathrm{B}$) reacts with ${\mathrm{OH}}^{-}$ by ${\mathrm{S}}_{\mathrm{N}}2$ mechanism.