The pH scale is a way of describing how acid or alkaline a substance is.
1 molar hydrochloric acid has a H+aq concentration of 1 (100) mol/l.                               Its pH  is  0.
Very pure water has a H+aq concentration of 10-7) mol/l.                                                    Its pH  is  7.
1 molare Sodium hydroxide solution has a H+aq concentration of 1 (10-14) mol/l.     Its pH   is 14.