What do you mean by common ion effect?

We know that weak acids and weak bases slightly ionize in water and equilibrium is established in their solutions. The phenomenon of suppression of the degree of dissociation of a weak acid or a weak base by the addition of a strong electrolyte containing a common ion is known as common ion effect.

For example: let us consider the ionization of a weak base ammonium hydroxide i.e. NH₄OH as:
NH₄OH (aq) ⇌ NH₄⁺ (aq) + OH^– (aq)

The ionization constant for the base will be:
K_b = [NH₄⁺] [OH^–] / [NH₄OH]
If solid NH₄Cl is added to the solution, the concentration of NH₄⁺ ions increases. According to Le-Chateliers’s principle, the equilibrium shifts to the left. As a result, the concentration of OH^– is considerably decreased and the weak base NH₄OH becomes even weaker in the presence of its salt.

NH₄OH ⇌ NH₄⁺ + OH^–
(Equilibrium shifts to left side)
NH₄Cl ⇌ NH₄⁺ + Cl^–
This phenomenon is known as common ion effect.