Bruno, Shmuel and the other inmates are stopped inside a changing room and are told to remove their clothes for a "shower". They are packed into a gas chamber, where Bruno and Shmuel hold each other's hands. A Schutzstaffel soldier pours some Zyklon B pellets inside, and the prisoners start panicking, yelling and banging on the metal door. When Ralf realises that a gassing is taking place, he cries out his son's name, and Elsa and Gretel fall to their knees in despair and mourn Bruno. The film ends by showing the closed door of the now-silent gas chamber, indicating that all prisoners, including Bruno and Shmuel, are dead.
I don't think it means anything, it is what it is, it's a poignant, lingering shot, the silence allows the viewer to be alone with their thoughts for a few moments and digest what has happened, the horror of it all, not just for the death of Bruno, but for all of those who were murdered in such a way. It's something that goes beyond the 'horror of war' cliche and allows us to imagine something more, that at that time, even amongst the accepted wartime horror, an even greater evil existed.