Opposing counsel are the two different sides to the case's legal representation collectively. That is, the plaintiff's legal counsel and the defendant's legal counsel would be referred to as opposing counsel because they are on the opposite sides of the case.
Opposing counsel are required to share information that relates to evidence, facts and witnesses in the case. Each side will request information from the other side until both sides of opposing counsel are satisfied that all evidence has been shared before the case proceeds to court.