Recall the equation of this equilibrium!
$Fe^{3+}$ $+$ $SCN^-$ $\rightleftarrows$ $Fe(SCN)^{2+}$ (disregarding the hydration of ions)
Why does the addition of $KCl$ shift the equilibrium to the left (merely visible)?
According to the principle of le Châtelier, the addition of a product tends to remove a portion of the products to reform the reactants.
Why does the addition of $Fe^{3+}$ or $SCN^-$ shift the equilibrium to the right?
According to the principle of le Châtelier, the addition of a reagent tends to remove some of this reagent to form products.