What does pH mean in practice?
The scale is logarithmic, so a change of one unit represents a tenfold change in hydrogen ion activity, not a linear difference.
When does it matter?
pH affects corrosion, metal solubility, chlorine effectiveness and deposit operation, but without alkalinity it does not describe pH stability.
Which solutions are related to it?
It is measured with a calibrated instrument and interpreted along with the temperature, alkalinity and process involved.