Poolish is a fermented batter, generally a mixture of water, flour and yeast which acts as the leaven
for certain kinds of bread dough.
There are distinct bread types, depending on the rising agent: yeast-raised bread, poolish bread, and leavened bread.
Although it doesn't have the full flavour of a leavened bread, poolish gives a bread with more flavour than one just using yeast.
.[Translator's note: the term poolish is more common in French baking than in English. For more information see the Pre-ferment, article on Wikipedia.].
Example: "Prepare a poolish by mixing the water, flour, yeast and a teaspoon of sugar."
Meaning: Mix the ingredients thoroughly but gently, preferably using a whisk, until the mixture is quite smooth.
The prepared mixture is covered and left in a warm place to ferment. This produces a rather sticky batter full of bubbles, the poolish, which can then be mixed with flour and water to make bread.