What does too high a Ph value in (drinking) water mean?

Answer

The pH is a measure of the concentration of H+ (hydrogen) in the water.

H2O <-> H+ + OH- : water is in equilibrium with its ions H+ and OH-

The pH is: – the logarithm of the concentration of H+

pH between 0 and 7 is an acidic solution (excess H+), pH between 7 and 14 is a basic solution (excess OH-). pH=7 is a neutral solution.

Of course, water should not be too acidic or too basic, so pH close to 7. I don’t know the limits within which the pH should fall for drinking water, but you can certainly look this up.