Pure water has a formula of H20. Water is able to dissociate into a hydrogen ion (H+) and a hydroxide ion (OH-). This occurs in a chemical reaction that is reversible. Both of these ions are very reactive. In equilibrium, the water is said to be neutral. The hydrogen and hydroxide ions are balanced.
The term acid is used to describe any substance that adds more hydrogen ions in a solution. This will cause the pH to become lower. An example is when you add HCl (hydrochloric acid) to water. The HCl will separate into H+ and Cl- ions. The additional H+ ions makes this an acidic solution.
Any substance that reduces the amount of hydrogen ions in solution is referred to as a base. An example of a base is the substance ammonia. When it is added to a solution, the ammonia attracts and bonds with a hydrogen ion in solution forming ammonium ion--NH4+. Some bases can dissociate to hydroxide ions (OH-) and accept the H+ ions of the acid to form water.
The pH scale is a measure of how acidic or basic a solution is. It actually measures the H+ concentration in a solution. Solutions which have more hydrogen ions are acidic, and those with more hydroxide ions are basic.
On the pH scale, 7 represents neutral water. As you proceed downward on the scale--6,5,4,3, etc., the solution is increasingly more acidic. Likewise, when you proceed upwards from 7, the solution is increasingly basic.
It is important to note that there is a 10 fold difference between each number in how acidic or basic a substance is. For example, an acid of pH 3 is 100 times more acidic than an acid at pH 5.
Cells require specific environments in which to carry out life processes. Any change in the acidity can affect chemical reaction rates in a cell. If pH values are too disrupted, a person may not survive. The body has buffers to regulate pH. These can be a weak acid and its corresponding base which will accept H ions when their levels are too high or can donate H ions when their levels are too low.
I have included a pH scale to visualize examples of acids and their pH values.