When we look at pH, we are assessing the acidity or alkalinity of the substance or fluid. Technically it refers to the concentration of H+ ions i.e.
pH = 1/ log [ H+ ]
When pH = 8 : There are 6 billion H+ ions in solution
When pH = 7 : There are 60 billion H+ ions in solution
When pH = 6 : There are 600 billion H+ ions in solution