pH is the measure of the acidity or alkalinity of a solution.
In pure water at 25 °C, the concentration of H+ equals the concentration of hydroxide ions (OH-). This is defined as "neutral" and corresponds to a pH level of 7.0.
The pH of different body fluids, including urine, saliva, and blood, varies with function and other factors. They are mostly tightly regulated systems to keep the acid-base homeostasis. The pH of blood is known to be slightly basic, at a value of 7.4.
pH is vital in maintaining the functioning of cells. For example, enzymes are heavily affected by changes in pH, and have an optimum pH at which they operate. Outside a small range they can denature and cease to catalyse vital reactions.