The starter culture refers to the soughdough starter, while it's still being made. In order for the starter to be able to leaven bread products, it needs to develop a very active microorganism culture, what we call 'wild yeast'. For this culture to stay alive and active, we need to 'feed' the starter constantly with water and flour at the same ratio. After a few days of constant feeding, the starter culture stars to bubble up and become very active, as a result of the release of gas and acids from the living creatures what were already in the flour or in the enviroment.