The solubility of a partially soluble salt is decreased when a common ion is added.
Consider the equilibrium established when acetic acid, HC2H3O2, is added to water.
At equilibrium H+ and C2H3O2- are constantly moving into and out of solution, but the concentrations of ions is constant and equal.
If a common ion is added, e.g. C2H3O2- from NaC2H3O2 (which is a strong electrolyte) then [C2H3O2-] increases and the system is no longer at equilibrium.
