Retarding dough is a process where you let it proof in a cold environment for a long period of time.
Almost all of Reinhart’s recipes call for using a starter of some kind. This basically involves reserving some dough from your last batch and letting hang out in the fridge for a day or two to be used as part of the next. The yeast continues to hang out and do its thing, and as a result the bread is much, much more flavorful.