Baking soda is sodium bicarbonate, an alkaline mineral salt used both as a leavening agent and, separately, as a household cleaning agent. In baking, it needs an acidic ingredient - such as buttermilk, yogurt, vinegar, or cocoa - to react and release the carbon dioxide that makes batter rise; without enough acid present it can leave a soapy, metallic aftertaste.
Beyond leavening, baking soda is used to tenderize meat, a common step in Chinese cooking where it's rubbed onto meat before velveting or stir-frying, and to raise the pH of water for a browner, crisper crust, as with pretzels or some fried chicken batters.