Consider the solubility equilibrium of a sparingly soluble salt, AgCl.
The solubility product, Ksp is given by,
Consider addition of a strong electrolyte AgNO with a common ion Ag+.
The concentration Ag+ in the solution is increased, hence by Le Chatelier’s principle the equilibrium of AgCl is shifted to left hand side since the value of Ksp is constant.
Thus in the presence of a common ion, the solubility of a sparingly soluble salt is suppressed.
