In practical terms it measures acidity on a scale of 1 to 14. 7 is neutral. Less than 7 is acidic while over 7 is alkaline. Soil acidity varies due to parent material, amount of rainfall, type of plants and agricultural practices. Soils in hot and humid areas are typically acidic.
Some dry climates are alkaline.
Local experts should know if the soil is normally acidic or alkaline. Different crops require different pH's for optimum growth. The majority of plants prefer soil between 6.0 and 7.0 pH. A major exception is the Ericaceae plants such as azaleas, rhododendrons and blueberries. Sweet means a low acid level. There is no technical definition but this would correlate with a pH of 6 to 7.