That's a sign that the screen has separated between the backplate and the frame. This is usually something we'll see when dealing with lower quality aftermarket screens.
You will have to pry the plate out of the metal clips along the edges, preferably with a plastic spudger-like tool. Be very gentle because the logic board is directly underneath and you don't want to cause any damage while the battery is still connected.
Follow this guide to give you some visual indications of what the backplate looks like from the other (hidden) side.