The motion means that the lender named as the plaintiff in the original complaint no longer owns the note and mortgage. The motion is to correct the court records by inserting the correct owner.
Disclaimer: This answer is provided for informational purposes only, does not constitute legal advice, and does not create an attorney-client relationship. Actual legal advice can only be provided after completing a comprehensive consultation in which all of the relevant facts are discussed and reviewed.
The Motion to Substitute Party Plaintiff is frequently a maneuver by the Plaintiff law firm to sneakily back out of an incorrect filing, where the wrong party was named as the Plaintiff at the outset of the action. However, unless you really know what is going on under the hood, there is no meaningful way to respond to this or to appropriately oppose it, even if it should be opposed.
You should consult an experienced foreclosure defense attorney to obtain specific advice regarding the defense potential as to the Motion to Substitute and to the case in general. Depending on all the details, it may be possible to successfully oppose the Motion and it may be possible to put on a viable defense to foreclosure. However, while people are allowed to represent themselves in court, it is a huge mistake to try. In order to do this effectively, you should obtain the services of knowledgable foreclosure litigation counsel. This is not something that even most attorneys know how to do. We have quite a few clients who are themselves attorneys, yet have come to us for help because they understand this, and realize that in order to have a chance against the "big guys" they need really qualified people to help them.
Doing this properly is very difficult and detailed work. It is a very specialized area of law which most attorneys do not fully understand, and there are few if any consumers who would be able to put together any sort of viable defense. I am puzzled at why any non-attorney would think he or she could successfully handle a litigated matter without an attorney highly qualified in the area of law in question. This is comparable to doing delicate surgery yourself or a loved one if you are not a surgeon. The law is very complex, litigation rules and procedures are very complex, the strategic considerations are very complex, and the area of foreclosure litigation is something even most attorneys know nothing about as explained above. Every case is different, what might be possible in this situation would depend on the exact details and legal issues in your case. Only a really qualified foreclosure defense will know how to even figure out what those are. An attorney who really understands how this works needs to hear all the details.
If you care how this turns out, I urge you to find an experienced attorney who is knowledgable about foreclosure defense to represent you. I see pro se people in court all the time. It is sad and frustrating to me - they are lost, cannot possibly know enough to be effective, and of course, they get walked all over, even when their issues are "winnable".