Solubility of AgCl in a solution of common salt is lower than in water. This is due to

Decrease in the ionization of weak electrolyte by the presence of a common ion from a strong electrolyte is known as common ion effect.

${\mathrm{AgCl}}_{\left(\mathrm{s}\right)}\to {{\mathrm{Ag}}^{+}}_{\left(\mathrm{aq}\right)}+{{\mathrm{Cl}}^{-}}_{\left(\mathrm{aq}\right)}$

${\mathrm{NaCl}}_{\left(\mathrm{s}\right)}\to {{\mathrm{Na}}^{+}}_{\left(\mathrm{aq}\right)}+{{\mathrm{Cl}}^{-}}_{\left(\mathrm{aq}\right)}$

Here Cl− ion acts as a common ion.