It was an insert shot that was originally supposed to go in the “Vader prayer” scene. It’s meant to be Vader’s ashes, and they were meant to have been stored in Ren’s quarters along with Vader’s helmet, but then they decided to have Ren keep his helmet on for the “prayer” scene and for some reason Abrams thought the shot fit during the interrogation scene.
So as it stands now, Ren apparently interrogates prisoners next to Vader’s ashes.