pH is the acronym for potential hydrogen. It is a measure of the degree of saturation of the hydrogen ion in a substance or solution. H3O (hydronium ion+) is the acid element and OH- (hydroxyl ion-) is the base or alkaline element. When the H3O and OH- are out of balance, a pH meter will detect this and the reading will move above or below 7 (neutral). If one goes up the other goes down and vice versa. In the human body, a pH balancing act is continuously going on to maintain homeostasis.
pH is actually an exponential scale. Every unit change in pH equals a 10-fold increase or decrease in acidity (or basicity). Thus, dropping from pH 7 to 6 is a 10-fold increase in acidity. However, dropping further to pH 5 is another 10-fold increase, or a 100-fold increase (10 x 10 = 100) in acidity from pH 7. For this reason, even minor deviations in pH around the more extreme values constitute much greater changes in acidity (or basicity) than seemingly major changes around the neutral mark. For example, merely dropping from pH 3.1 to 3.0 equals an increase in acidity more than 238 times greater than dropping from pH 7 to 6.