Definition: a raising agent (sodium bicarbonate) that reacts with acid and heat to produce carbon dioxide, encouraging spread and browning.
Bicarbonate of soda — “bicarb” — needs an acid to activate (think buttermilk, brown sugar, cocoa or lemon). In cookies it does two jobs: it helps them spread, and it raises the pH so they brown faster and taste toastier. It’s strong stuff, so it’s measured in small amounts; too much and you get a flat, soapy, slightly metallic result. It isn’t the same as baking powder, which has its own built-in acid.
In practice: measure carefully and pair with an acidic ingredient. If a recipe has no acid, it probably wants baking powder instead.
Related:the Maillard reaction,creaming.