One of several pH testers available at Garden Centers. This one also tests Fertility. The needle is at rest on the Neutral "7". Submerged in Alkaline (sweet) or Acidic (sour) soil, the needle would move in the appropriate direction proportionate to the degree of acidity.
In use, you generally take a soil sample and mix it with water to get a paste. Then the probe(s) are stuck into the paste for the suggested time period, and a reading is taken on the dial. This reading allows you to adjust the soil pH for the particular plants you have in that area. It is best to have plants with similar pH preferences in a given area to simplify soil Amending procedures.