This is a plane that is directly in between the two planes.
Work Step by Step
While this plane is fairly hard to draw, it is not too difficult to describe. Recall, a locus to the plane is the same distance away from the plane at all points. Since there are two planes, the locus must be the same distance away from either plane, as the book specifies. Thus, the locus is the plane that is directly in between these two planes that is parallel to both planes.