Poolish is a fluid preferment, typically made with equal weights of flour and water plus a small quantity of commercial yeast. It ferments before the final dough is mixed, developing aroma and improving extensibility.
A mature poolish is bubbly and may just begin to recede at its surface. Yeast quantity depends on temperature and the planned schedule. Unlike biga, it is generally liquid; unlike a sourdough levain, it need not contain a maintained sourdough culture.
Reference: Further reading.