what is Solubility Product?

For sparingly soluble salts(solubility<0.01mol/L), it is experimental fact that the mass action product of the concentrations of the ions is a constant at constant temperature. This product Ks is called the solubility product. For a binary electrolyte

AB=A⁺ + B^-  then Ks_(AB)=[A⁺][B^-]

In general

For electrolyte ApBq, which ionizes into pA^q+ and qB^p- ions, then

ApBq = pA^q+ + qB^p-

and solubility product K_s(ApBq) = [A^q+]^p[B^p-]^q

Lets discuss your question: PbCl₂ ionises to give Pb⁺ and 2Cl^- ions

PbCl₂ = Pb⁺ + 2Cl^-

Ks = [Pb⁺][Cl^-]²