While there are many different levels to measure within your pool, the three most important levels to keeping your swimming pool chemistry balanced are, total alkalinity, pH, and calcium hardness.
You will probably not change the water in your pool for many years. Continuous filtration and disinfection removes contaminants which keep the water enjoyable but this is does not balance your water.
A pool that is “balanced” has proper levels of pH, Total Alkalinity, and Calcium Hardness.