Levain - natural yeast starter
Levain is a natural yeast starter, an ancient way of making bread rise. The starter is made solely from flour and water and left to ferment in the air; the baker watches it and "feeds" it regularly with equal amounts of flour and water as it is used up. The starter can be given a personal touch by using, for example, a little fruit juice or beer. Bread made with a natural yeast starter has a distinctive taste. It is slightly tangy and denser, with a flavor unlike that made from ordinary baker's yeast. 

Levain-chef or Chef 
Chef is a piece of fermented dough that is set aside from the day's batch of bread dough to be used to leaven the next day's bread. Its volume is progressively increased in the interval between production times so that it is big enough to "seed" the next day's dough. The chef can range from 10-20 lb. 

First starter (Levain de première)
When used, the dough will rise well, neither too young (insufficient rising power) or too old (overly sour). The weight of the chef is doubled or tripled by adding more water and flour. The levain is refreshed or renewed, resulting in the "levain de première."

Second starter (Levain de seconde)
Six or seven hours later, a new enrichment. After kneading, the second starter is obtained which ferments for 4-5 hours. 

Ready to use
After this period, there is a final addition which leads to the starter being ready to use. It ferments one to two hours more before the dough is prepared and kneaded.