The common ion effect is a specific application of Le Chatelier's principle. Adding an ion that is already present in the equilibrium shifts the equilibrium to reduce that ion's concentration. For weak acid HA in water: HA <=> H+ + A-. Adding NaA (source of A-) shifts left, reducing [H+] and suppressing dissociation. This means the pH of 0.1 M CH3COOH + 0.1 M CH3COONa is higher (less acidic) than 0.1 M CH3COOH alone. For Ksp: adding a common ion drastically reduces solubility. PbCl2 in 0.1 M HCl: Ksp = [Pb2+][Cl-]^2, and [Cl-] is dominated by HCl, so [Pb2+] = $\frac{Ksp}{0.1}$^2.