The text seems to refer to creating pressure on teeth, either to move them, or to prevent them from moving. I interpret at a dental school often, and that is what I make of that sentence.
When, for example, an upper molar is removed, the corresponding lower molar loses the pressure it would receive when the patient bites. Thus, over time, that lower molar progressively "grows" (moves) upward and becomes less stable and subject to problems. The solution is to place an implant in the space of the missing tooth, or create a removable device that can "bite" against the lower tooth and keep that one normal by opposing it. Clearly, this insurance company is specifying that it will cover the cost of this "appliance" to maintain the surrounding teeth in proper health.
I disagree completely with the interpretation of "opposing" as meaning "on the opposite side" in this case.