Soil pH is a measure of the acidity or alkalinity of the soil and is expressed by a number on a scale from 1 to14. A neutral reading is that of 7. Readings below 7 indicate acid conditions - the lower the number, the more acidic. Those numbers above 7 indicate increasing levels of alkalinity. Although optimum soil pH varies from one species to the next, a range of 5.5 to 6.8 is generally considered best for most plants. 

Additional Resource
http://edis.ifas.ufl.edu/SS480