pH is the measure of your waters acidity. If it is too low it is corrosive and will eat at you plaster/vinyl liner, etc., if it is too high it will cause scaling and your water will have a tendency of being cloudy.
pH and chlorine have an inverse relationship. The lower your pH, the more efficient your chlorine will be. For example, if your pH i8.0 and you put 10 pounds of chlorine in your pool, you are only really using 2.1 pounds. Now if your pH is 6.5 and you dump 10 pounds of chlorine in your pool, then you are using 9 pounds.
The pH of a persons eye is 7.5. Keep your pH at that number and you will have little or no irritation to your eyes.
But, when the pool is green and no one is swimming anyway,
drop your pH to 6.5 and shock it. This will give you the most bang for your buck. Plus, since shock has a pH of 11.8, your pH will end up pretty close to the desired range once it’s ready to swim.