What is common ion effect? Give an example.

The reduction of the degree of dissociation of a salt by the addition of a common-ion is called the common ion effect.
E.g.: In a saturated solution of silver chloride, we have the equilibrium:
$AgC{l}_{\left(aq\right)}\rightleftharpoons A{g}^{+}+C{l}^{-}$
When sodium chloride is added to the solution the concentration of $C{l}^{-}$ ions will increases. The equilibrium shown above will be shifted to the left to form more of solid $AgCl$. Thus, the solubility of $AgCl$ will decrease.