pH is a measure of the relative concentration of hydrogen (H+)  and hydroxide (OH-) ions in an aqueous solution.

The scale is 0 - 14 standard units. A reading of 0 - 7 is acidic; 7 - 14 is basic (or alkaline), and 7 is neutral.

Acidic water, along with low hardness (soft water), tends to be corrosive to your water pipes, potentially dissolving lead and copper into your tap water.

Basic water is not itself is not a problem, though it may have a bitter taste.

Alkalinity, which is a separate parameter, is your water's buffering capacity against changes in pH.

Health Limits

USA Primary Drinking Water Standard limit: 6.5-8.5

Resources

Maine Geological Survey / Ryan Gordon, "Corrosive Water – Facts, Common Questions, and Resources" (2017, 8 pg. pdf) Wikipedia, "pH"

MEL Test Methods for pH

Drinking Water –  SM 4500-H+ B (SDWA Compliant)
Wastewater –  SM 4500-H+ B (NPDES Compliant)
Solids –  EPA 9045D (SW846/RCRA Compliant)
AQ RCRA - EPA 9040C (SW846/RCRA Compliant)

Sample Requirements

Drinking Water:

Container: plastic (recommended), glass
Volume: 125 mL
Hold Time: 15 minutes (MEL will run pH as soon as received with notation)
Preservation: fill container to top recommended

Wastewater:

Container: plastic (recommended), glass
Volume: 125 mL
Hold Time: 15 minutes (MEL will run pH as soon as received with notation)
Preservation: fill container to top recommended

Solids and Sludges:

Container: plastic (preferred), glass
Volume: at least 25 grams preferred
Hold Time: n/a, but ASAP recommended
Preservation: n/a, but fill container to top recommended