Poolish
A poolish is a liquid preferment of equal weights of flour and water with a very small amount of yeast, mixed several hours before the main dough. At 100% hydration it has the consistency of a thick batter.
Its purpose is to move part of the fermentation ahead of the mix: flavour and aroma develop, organic acids build up, and gluten partly forms on its own. Bread made with a poolish is more aromatic and stales more slowly than bread mixed in one go.
You judge it by eye: the poolish rises, covers itself in bubbles and begins to dip slightly in the centre, which marks the peak. Timing depends on the yeast quantity and temperature, typically 8 to 16 hours at room temperature.