A leavening agent which reacts when combined with an acid such as buttermilk, yogurt or molasses producing carbon dioxide bubbles which cause a dough to rise. If such a thing as 'baking soda' does not exist in your country, look for it at the chemist's, where it's usually known as sodium bicarbonate. To test baking soda for freshness: Add 1 teaspoon of baking soda to 1/2 cup of hot water. The soda will actively bubble if it is fresh.
Baking soda reacts immediately when wet, so it should always be added to dry ingredients first. Once the batter is wet it should be put in the oven immediately.