Baking powder
Baking powder is a compound powder made of a carbonate and an acid. Some kind of starch is added as a stabiliser.
The carbonate, when combined with the moisture from the dough and the acid, will ensure the formation of carbon dioxide during baking. This gas is what will cause the dough to rise.
Tip: if you want to check if your baking powder is still good to use, drop a small spoon of the powder in warm water. If you see it bubbling, everything is fine.