pH is a scale of numbers from 1 through 14 that is used to indicate the acidity of a water resource. Water is said to be acidic if the pH is below 7, and basic when the pH is above 7. A pH value of 7 is considered neutral, which means it's neither acidic or basic. Waterbodies in the low end of the pH scale are of particular interest to scientists concerned about the effects of acid rain on aquatic plants, fish and wildlife.1
"pH" stands for "potential of hydrogen". A pH measurement has no units associated with it. The 0-14 pH scale is logarithmic and inversely indicates the concentration of hydrogen ions in a solution.
1(Definition taken from LAKEWATCH Circular #101: A Beginner's Guide to Water Management — The ABCs: Descriptions of Commonly Used Terms)