PH is a measure of how acidic or basic a solution is. The PH scale measures from 0 to 14. The middle of the scale is 7, which is natural. Water is 7 on the PH scale.
A number greater than 7 on the PH scale is rated basic or alkaline.
A number less than 7 on the PH scale is rated acidic.
One of the most acidic substances is battery acid, which ranks about 0 on the scale. Very acidic!
One of the most basic substances is lye, which ranks about 14.
The second link has a chart of some substances.