A solubility product is a convenient way to represent the low solubility of a sparingly soluble compound. For salts K_nA_m the solubility product

L = [K]ⁿ·[A]^m

The unit is M^n+m, where M is mol/L. (Activities are approximated by molar concentrations).
pL is defined as -logL. Values are positive. A large positive value means a very low solubility of the salt.