The pH is a measure of the acidity of the water.
The pH scale goes from 0 to 14, where pH 7 is neutral. If the pH is above 7, the water is basic; if it is below 7 the water is acid. The optimum pH for pool water is 7.4, since this is the same as the pH in human eyes and mucous membranes. A pH of 7.4 also gives good chlorine disinfection.