Solubility product

The product of concentrations of ions in saturated solution of an electrolyte at a given tempera­ture is constant and is known as solubility product. For silver chloride, the equilibrium is formulated as follows.
Undissolved salt ⇋ ions in solution
AgCl(s) = Ag+(aq)+ Cl (aq)
K = [Ag+][Cl-]/[AgCl]
The concentration of the undissolved salt is constant (say k) at a particular temperature regardless of the amount of solid silver chloride present. (This is true for any solid that its concentration is independent of the amount of solid), we have:
K = [Ag+][Cl-]/k    OR       K.k = [Ag+][Cl-] = ksp
The constant ksp is called as the solubility product.

Common ion effect. If the solution of a strong electro­lyte is added to that of a weak electrolyte having a common ion, the ionization of weak electrolyte is suppressed more and this decrease in the degree of ionization of the weak electrolyte in solution on being mixed with a strong electrolyte containing
a common-ion is termed as common ion effect.
For example ionization of NH4OH is suppressed by the addition of NH4CI which provides common NH4+ ion.