Maybe the OP means "pH"?
"pH" stands for potential Hydrogen. It is the "Standard" chemical scale (the Sorensen Scale) for measureing the relative balance between acidity and alkalinity. It ranges from 1 (highly concentrated acid) to 14 (highly concentrated alkyline), with a pH of 7 being about neutral.
The pH balance is an important factor in growing because it affects water, nutrients and soil (or hydroponic medium) - and can cause serious trouble, if not kept in line!