You have probably at least heard of or seen the term pH. It has become something of a household word. In advertising, there has been mention of things like "the pH of your skin" and "pH of your diet." You may also remember experiments in school using litmus paper to see if a solution was acidic or alkaline.
So, pH is just a unit to represent one characteristic of a solution, just as the meter is a unit of length.
So, what kind of characteristic is represented by pH?
Lemon juice tastes sour. Soapy water is slimy and tastes somewhat bitter. If you dip blue litmus paper into lemon juice, it turns red. If you dip it into soapy water, it turns blue. Because lemon juice is acidic, it makes blue litmus paper turn red.
Because soapy water is alkaline, it turns red litmus paper blue.
And pH represents the acidity or alkalinity of a solution, such as your lemon juice or soapy water.
In the chart below, you can see that pure, neutral water has a pH of 7. Any solution with a pH below 7 is acidic, and if the pH is above 7, then it's alkaline.