pH

pH is technically defined as:
pH = -log[H+]

pH monitors the concentration of free protons in solution. It is frequently used to determine whether an environment is "acidic" (pH < 7) or "basic" (pH >7). The pH scale ranges from 0-14, where 0 is the most acidic and 14 is the most basic. pH is important to measure in beer and kombucha because it can provide insight into microbe health and fermentation progress. 

There are two main ways of measuring pH: colorimetric and potentiometric measurements. Indicator molecules are used in the colorimetric measurements. The color of the indicator molecule is affected by the pH of the environment (phenolphthalein is a good example). pH meters are frequently used to perform potentiometric measurements.