Biga is a yeast pre ferment, or a yeast starter
. It is of Italian
origin and is a bit more dry compared to a levain-levure
or a poolish
It is commonly used in a ciabatta though it can also be used for foccacia and other breads of italian origin.
Like most yeast pre ferments, it not only leavens the bread but also imparts a special flavor that can not be achieved when using "no time" yeasts like instant dry yeast. The yeasts co-exist with bacteria, these bacteria give off lactic acid and acetic acid, the latter being the reason for the pleasant sourness in a sourdough bread.