what does plantiff's motion substituting party plaintiff mean

Not having seen it, I can’t say for sure, but I can generalize from experience.  Most likely, the original Plaintiff sold their interest in your Note, and, therefore, the new Plaintiff needs to move the Court to be substituted as the new Plaintiff.  While there is some case law out there that gives some room to argue against this motion, most likely the Court will grant the motion.  If you are curious what it means to your case, it means that you will be dealing with a new Plaintiff (assuming my generalization meets your facts).  Unfortunately, I can’t make guesses as to what it means substantively to your case.  It’s possible that the substitution could present defenses or it may not mean much.  As far as procedurally, it just means the old Plaintiff is out and the new Plaintiff is in.  It may also mean you’ll be dealing with new attorneys on the other side.  If you are defending your foreclosure without an attorney, I would highly advise at least a consultation so you know exactly where you stand.  Be wary of “self-help” sites out there.  The litigation is far more complicated than it may seem, and you need to have the whole picture.  This motion alone may mean a lot or it may have little impact – without all of the details, unfortunately, I can’t provide more guidance.