pOH is the lesser known cousin of pH. Unlike pH, which measures the concentration of H3O+ ions, pOH measures the concentation of OH- (hydroxide) present in a given solution.
Numerical value for pOH can be determined the same way as one determines the value of pH: simply take the negative log of the concentration of OH-.
Working conversely: to try and find concentration of OH- from a given pOH value, 10-pOH= [ OH- ]
You can also determine pOH by keeping in mind the fact that, by definition, pH + pOH = 14 (the maximum value for pH or pOH is 14).
If a solution has a low pOH (anything less than 7) it has a high concentration of OH-. If a solution has a pOH (or a pH) of 7 it is neutral. If it has a high pOH it has a low concentration of OH- and a higher concentration of H3O+ ions.
The inverse relationship between pH and pOH is such because Kw (which is equal to 1.0 E -14) = [ OH- ] [ H3O+ ].