A biga is an Italian preferment that refers to any style of preferment that contains flour, water and yeast, or in this case, Motherdough. 

It is more common for a biga to have less hydration than a poolish. 

A biga takes longer to finish fermentation as compared to a poolish containing the same percentage of Motherdough. This is because yeast’s movement is impeded by lower hydrations, taking them longer to propagate and consume all the starches contained within the bread dough. This is why biga preferments will usually, but not always, contain more Motherdough than a wetter style of preferment like a poolish.