Acetic acid dissociates as CH3COOH  CH3COO– + H+. If a little amount of sodium acetate is added to its aqueous solution

CH3COOH  CH3COO– + H+CH3COONa is added, CH3COONa is a strong electrolyte. It exists in ionized form. Thus addition of CH3COONa increases [CH3COO-]. Addition of products favour backward reaction. Thus dissociation of CH3COOH is supressed.