When the defendants attorney files for a Motion to Compel, what exactly is this type of hearing?

It is a hearing held so the Judge can hear the reasons the Plaintiff is not either answering Interrogatories or producing requested documents. The Judge usually will give the Plaintiff 20 additional days with which to answer the interrogatories or produce the requested documents. If the Plainitiff does not comply with the Judge's order then another hearing will be held. The hearing is Motion for Contempt. At that hearing the Judge will either impose sanctions on the Plaintiff or the case can be moved to Dismiss the suit.