Mash pH is the acidity of the mash itself, ideally sitting somewhere around 5.2-5.6 (measured at room temperature) for most beer styles — outside that range, enzyme activity during conversion becomes less efficient, and the finished beer can pick up a harsh, astringent edge or an unusually dull, flat character depending on which direction pH drifted.
Grain bill and water alkalinity pull mash pH in opposite directions
Darker roasted malts are naturally acidic and push mash pH down; a very pale grain bill, with little of that natural acidity present, tends to leave mash pH sitting higher unless the water itself has low alkalinity to match. Water bicarbonate (alkalinity) works the opposite direction, buffering against that acidity — which is exactly why Dublin's naturally high-bicarbonate water suits a roasted-barley-heavy stout mash so well, while that same water would likely push a very pale lager's mash pH uncomfortably high without correction.
Measuring it, not assuming it, is the only reliable approach
Because both grain bill and water chemistry affect the outcome together, the only genuinely reliable way to know a mash's actual pH is to measure it directly with a calibrated pH meter or test strips rather than assuming a recipe's stated water profile guarantees a specific result — real municipal water varies enough from any published reference figure that a measured check is worth the extra few minutes.