Water balance is simply the relationships of different chemical parameters to each other. Your pool water is constantly changing. Anything and everything directly and indirectly affects water balance - from sunlight, wind and rain to the oil, dirt and cosmetics which may enter the pool.
Continuous filtration and disinfection remove contaminants which keep the water enjoyable, but this is not water balance. A pool that is "balanced" has proper levels of pH, Total Alkalinity and Calcium Hardness. So then, balanced water is that which is neither over or under saturated. Water which is under saturated will attempt to saturate itself by dissolving everything in contact with it in order to build up its content. Water which is over saturated will attempt to throw off some of its content by precipitating minerals out of solution in the form of scale.
