Biga is an Italian leavener mixture made from flour, water and yeast. It is made up each time for a loaf of bread — it is not meant to be reserved and carried over, as is done for a sourdough starter.
The biga is the first part of the ingredients for your bread, then to it you add more flour, water, etc. It provides good flavour to bread, and helps the final product to retain moisture and freshness longer.
The definition of Biga has changed from what it used to be. It was originally made as a stiff leavener to reinforce the strength of weaker flours that were produced in Italy. It would be fermented about 18 hours in a place cooler than normal room temperature (though not in a refrigerator.)
Today, though, with stronger flours being made from North American wheat, a really stiff biga wouldn’t be desirable. It would make a bread dough too inelastic. Consequently, biga has become more of a generic term, with no standardized definition of whether the leavener mix should end up liquid or firm, sweet or fermented till very tangy, etc.