The Ion Product Constant is a term for the ratio between OH- and H+ in a solution. It's important in the study of kinetics for compounds. The Ion Product Constant is defined as the equilibreum constant times the concentration of the solution. For example, in H2O the ion product constant is:

Kw={H2O} * Keq
(55.6 M) * (1.8 * 10-16M)

It also should be noted that for water this is equal to the concentration of hydrogen ions times the concentration of hydroxide ions. Thus, if the concentration of hydrogen goes down, the concentration of hydroxide must go up, maintaining the constant.

Ion Product Constants will give you amphoteric compounds.