pH is the measure of the acidity of the water. The lower the pH the more acidic it is and the higher the pH the more basic it is. Maintaining pH, or potential Hydrogen, is simple and easy when you understand just a few basics. pH ranges from 0-14 with 7 being neutral.
Your pool or spa's pH should be between 7.2 and 7.6. Your eyes pH is between 7.3 and 7.4, so the closer you are to 7.4 the less burning sensation you will have in your eyes. If your pool or spa's pH is below 7 then the water becomes more corrosive and will start to wear away your metal components. For example your pump, your heater, and it also removed the plasticizers from the vinyl liner & pits the concrete, etc.
If your pool or spa's pH is too high then it will start to build up scale. Scale can begin to form on the warmest parts of the pool, so the heater is the first victim. Scale acts as an insulator on the heat exchanger. That can run up the cost of operating your pool because you have to use more natural gas or electricity. And finally the higher the pH is in your pool or spa the less effective your chlorine will be, and the less sanitized it will be.