Biga is an Italian preferment made from flour, water and a little yeast, then fermented before mixing the final bread dough. It is commonly stiff, although formulas vary. It is used in breads such as ciabatta to develop flavor and dough characteristics.
Biga is not simply a sourdough starter under another name. Follow the specified water content and ripening schedule; substituting a liquid poolish changes the final dough's hydration.
Reference: Further reading.