Water that is too Acidic or too Alkaline will cause chemical reactions that you don't want. The level of Acid and Alkali is your pH measurement.
Too acidic and it will cause metal corrosion, etching on surfaces of materials and eye irritation. It will also cause Chlorine to burn off at a faster rate.
Too Alkaline and it will cause scaling on the surfaces of the pool and plumbing. It may turn water cloudy and will also inhibit Chlorine from destroying pathogens.
Optimum pH levels are between 7.3 and 7.8. Zero would be extremely acidic, 14 extremely alkaline, while 7 is a neutral state.
The natural level for pH in the human eye is around 7.3, so the nearer to this level the more comfortable for your eyes.
To Lower pH you would need to add pH Minus (Muriatic Acid or Sodium Bi-Sulphate).
To Raise pH you would need to add pH Plus (Sodium Carbonate or Sodium Bicarbonate).
The balancing of the chemicals in your pool is an ongoing process because new elements are continuously being added in to the water, oils from skin, leaves and dirt falling in, a shot of chlorine etc. So regularly (weekly) testing your levels and keeping on top of them means you should have relaxed hassle free swimming.