pH refers to the ability of the water to be more on the acidic side of the pH scale or on the basic side of the pH scale.  Water, pure water, that is, has a pH of 7, which is right in the middle of the scale, from 1 to 14.  The lower the number, the more acidic, the higher the number, the more basic.  Beans, when germinating, prefer a pH level commensurate with what the mature plant best grows in, which is a range of 5.8 to 6.5, placing it on the slightly acidic side of the scale.  If the water is too acidic, with numbers lower than 5.8, this could eat away at the delicate structures of the bean seed as it germinates, resulting in an early destruction of the infant plant.  If the number of the pH is higher than 6.5, going towards the basic side of the scale, this could open the young plant up to all kinds of diseases that would be detrimental to its germination and establishment as a developing plant.  pH is a very important factor to consider in the success rate of beans in the germination process.