Opposition surge is an optical effect of rough surfaces. On just about every scale, this comet has a rough surface. The little lumps and bumps of topography on a rough surface cast little shadows on other visible parts of the surface. When you view a rough surface from a distance, you can't see any of those little individual shadows, but together they have an effect of darkening the surface. But that changes when the light source is directly behind you. You can't see any shadows, because they are being cast underneath all the surfaces that you can see. The surface at the subsolar point appears brighter because you can't see the shadows anymore. By measuring how large the opposition surge is, and the area over which it is visible, scientists can deduce some of the physical properties of the surface at scales much smaller than can actually be seen in photos.

Opposition surge is visible when you are observing an object at a phase angle (the angle from light source, to object, to observer) of close to zero degrees. Here's a little visual explainer of phase angle for another object that shows an opposition surge, Saturn's moon Rhea. I made this diagram for a longer post explaining phase angle.