Self-rising flour is flour that has been packaged with baking powder and salt already mixed in. Because it already includes baking powder, a leavening agent, self-rising flour does not need any other leavening agents added in order to make bread and other kinds of baked goods.
Leavening agents are substances used in baking that, under the right conditions, generate a large amount of air very quickly. When added to a dough or a batter, this sudden burst of air fills the dough with air bubbles and so causes it to rise (this rising, in turn, is what makes baked goods light and fluffy). Baking powder is a chemical leavening agent. It contains equal proportions of an acid and a base in powder forms, and when these powders are exposed to moisture, they chemically react with one another, producing large amounts of carbon dioxide gas. This is what makes self-rising flour "self-rising."
Bakers can make self-rising flour at home by mixing in 1 1/2 teaspoons of baking powder and 1/4 teaspoon of salt for every 1 cup of all-purpose or low-protein flour the baker needs. King Arthur Flour, an American company more than two centuries old and founded in Boston, sells self-rising flour in single 5-pound bags or sets of eight 5-pound bags as of June 2015.