The pOH of a solution is calculated by taking the negative logarithm of the hydroxide ion concentration. In simpler words, you look at how many hydroxide ions are present in the solution and then apply the logarithm to convert that number into a more convenient scale.
If the solution has a high hydroxide ion concentration, the pOH value will be low, meaning the solution is strongly basic. If the hydroxide ion concentration is low, the pOH value will be high, meaning the solution is less basic.
Once the pOH is known, you can also use it to find the pH, since both are connected.
In short: To calculate pOH, you use the hydroxide ion concentration and convert it into a logarithmic value that shows how basic the solution is.