In layman's terms, a Mother Sauce is, in some respects, the foundation of your sauce. Once you have the foundation, you can build off of it and create anything you want.  For example, the velouté sauce I made here is not necessarily something I would plainly serve on a dish.  While it tasted lovely, it clearly needed a little something more to be elevated to greatness.  A sauce veloute can be transformed into a beautiful mushroom sauce or as the base for a wonderful soup.