pH, "potenz hydrogen" is a measure of hydrogen ion concentration, i.e. [H_3O^+].
By definition, pH = -log_10[H_3O^+], so that acidic solution have low (or even negative) values of pH, and alkaline solutions have high values of pH.
In water, the following equilibrium occurs:
"H_2O(l) rightleftharpoonsH_3O^+ + HO^-
At 298*K we can measure the value of this equilibrium very accurately:
[H_3O^+][""^(-)OH] = 10^-14
And taking log_10 of both sides:
log_10[H_3O^+] + log_10[HO""^-] = log_10(10^-14) = -14
On rearrangement, 14=-log_10[H_3O^+] -log_10[HO""^-]
To give (finally):
14=pH+pOH