Scientists measure pH on a logarithmic scale that ranges from 0.0 (most acid) to 14.0 (most alkaline), with 7.0 being neutral. Don't worry if you can't recall the definition of logarithm; all you need to know is that each unit on the pH scale represents a tenfold difference in acidity or alkalinity. For example, if lemons have a pH of 2.0 and oranges have a pH of 4.0, this means that lemons are 100 times more acid than oranges.
You're probably wondering what all this chemistry has to do with your soil. Plant nutrients must be dissolved in the soil solution in order for plants to absorb them, and the pH of soil affects the solubility of minerals. Most vegetables and landscape plants grow best in soil with a pH of 6.0 to 7.5. When soil pH falls below 6.0, nitrogen, phosphorus, and potassium are less available to plants; and when the pH rises above 7.5, iron, manganese, and phosphorus are less available.
Each individual plant has an ideal pH range. Some plants, such as camellias and blueberries, have an ideal pH range that falls on the acid side, while other plants, such as daylilies and hollyhocks, grow well in soils that range from slightly alkaline to slightly acid. If you have soil with a very high or low pH, you can alter it. But before you ever add an amendment, make sure to get a soil test and follow the lab's amendment recommendations. Liming (adding limestone) increases the alkalinity of soil, and adding elemental sulfur (buy only sulfur products approved by the Organic Materials Review Institute, or OMRI) increases the acidity of soil. Regular applications of compost have been shown to help bring soil pH into balance. And, of course, you can also work with what you've got and grow plants that thrive in the natural pH of your soil.