For more flavoursome breads, bakers often use lightly fermented dough in their breads. These doughs are known as starters. The most well known form of starters are sourdough starters, or leavens. This type of starter is made by fermenting flour and water and using yeast that is naturally present in the air and can take a few days to come to its proper balance of flavours. But there are alternatives that take less time.
A poolish is a type of starter used in baking that helps bring greater complexity of flavour to breads. It is not as complex as perhaps a leaven or sourdough starter, but aids in the making of better tasting breads. It is generally made of equal parts water and flour, with a small amount of yeast added to the mix.