Pâte fermenté is translated as "old dough." It is literally a finished dough that is added to the freshly mixed dough. This may seem odd, but the practice probably started when home cooks had to save a piece of the old dough to make sure they had leavening for the next days bread. It can either be made especially for whatever bread you're making, or you could save a portion of the dough you make today and use it in tomorrows loaf.
Since it is an old dough, pâte fermenté does contain salt, in addition to the flour, water, and yeast. That's fine because the dough has still been able to develop the yeast and organic acids over the fermentation period. The pâte fermenté must be refrigerated overnight, and you can add it either cold or at room temperature to the new dough. Because the gluten has already been developed as well as the yeast and acids, the pâte fermenté should be added toward the end of the mix. That way, the gluten in the pâte fermenté won't get over developed.
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