Pre-Ferment ( Old dough) is just that : a piece of while flour that is reserved after mixing and incorporated into the next batch ( Final dough) of bread.
This is nothing difficult to make this kind of bread. You just mix and stir the flour and water and a bit of yeast, then leave it at your kitchen counter for overnight. In the next morning, you can mix the preferment dough and the final dough to bake bread. You will taste the flavor benefits of using some Pre-Ferment (old dough) in the new mix.
Here is the basic type of yeasted pre-ferments. ( References : ” Bread” by Jeffrey Hamelman and Wikipedia)