A levain is just a mixture of flour, water, and starter. It is a premix that will be used later in the dough you make, instead of using just your starter. You are taking a portion of your starter and putting it in another jar, feeding that other jar until it is ready to use. Then you will use all of that second jar for your bread-making. There are some benefits to this method.
One: It is a pre-ferment, which means it can alter the flavor.
Two: It allows you to bulk an off-shoot of your starter to make more than one bread without bulking your actual starter.
