When AgBr is dissolved in water, the following equilibrium is set up in solution;
AgBr(s)⇄Ag^+(aq) + Br^- (aq)
If we dissolve NaBr in the water, a common ion (Br^-) is now introduced into the system. This increases the concentration of Br^- and favours the reverse reaction hence more AgBr is precipitated. This is known as common ion effect.
