The pH scale is a measure of acidity. It represents how many free hydrogen ions are present in a solution.
Each level of [H+] hydrogen ion concentration on the pH scale differs by a factor of 10.
Pure water has a hydronium concentration of 1.0 x 10-7 one times ten to the power of minus seven moles per liter. This is pH 7, which is neutral. Solutions with a pH lower than 7 are acidic, whereas solutions with a pH higher than 7 are alkaline.
pH can be measured using a pH meter, or a pH indicator.
Do you want to learn more? Check this out:
Calculating pH
The pOH scale