pH is a measure of the acidity of a solution. Soil pH is important. A soil pH of  6.5 is ideal.(5.9 to 6.8) Carrots tend to have growth issues if your soil is too high in acid.  Note: lower the number the higher the acid. 

Note: battery acid has an approx pH of zero, lemon juice appox. pH of 2.1, beer approx pH of 4.5, water has a pH of 7.0, and milk has a pH of 6.8