What does the ending to The Boy in the Striped Pajamas symbolize?

The ending to The Boy in the Striped Pajamas symbolizes the terror and the brutality that defined the Holocaust.

In the film's final sequence, two separate events are simultaneously shown. Bruno and Shmuel are being herded along with hundred of other prisoners. Neither one is able to stop the mass of people moving them.  Bruno is comforting Shmuel with explanations such as "They want to keep us here from the rain."  While this is happening, Bruno's mom interrupts Bruno's father in a meeting to tell him their son is missing.  Bruno's father orders the Nazi soldiers to find his son.  They make their way to the fence where they find Bruno's clothing.  His father and the soldiers cross the fence and rush into the camp to try to stop what he fears has happened to his son.  Bruno's mother and Gretel lag behind, stopping at the fence.

During the pursuit, the prisoners are told to undress as they make their way into the gas chamber.  Bruno holds Shmuel's hand as the lights go out and the gas pellets are released.  With that, the door is shut and locked, while people are banging from the inside, trying to free themselves.

Looking for his son, Bruno's father comes across a set of emptied out barracks. He calls out his son's name to no response.  At this moment, Bruno's mother realizes what has happened to her son.  She lets out a harrowing and heart-wrenching cry.  Sobbing, she clings to her dead son's clothes as Gretel is stunned into silence.  Bruno's father hears his wife's cries and stares out, standing on the threshold of painful revelation. The film's final scene pans back, showing the door of the gas chamber.  No sound can be heard as the backwards panning camera shows the "striped pajamas" that the prisoners had to remove before entering.

The gas chamber door symbolizes the Holocaust.  The gas chamber itself is one of the most identifiable symbols of the time period.  It was the instrument that the Nazis used to carry out their plans of wiping out those deemed as unworthy of living.  The door symbolizes the immediacy of this slaughter.  Millions entered doors like the one shown in the final scene of the film.  None of them would leave.   The way the camera captures the silence and the "striped pajamas" left behind reminds us of this haunting reality.

The terror and brutality symbolized in the film's final scene is enhanced by the sobbing cries of Bruno's mother and the revelation in his father's facial expressions.  Bruno's father realizes that what he did for a living had direct consequences.  His son died because of his work.  Bruno's mother weeps because she knows that her complicity with her husband's work led to their son's death.  The emotional weight of two parents' understanding about Holocaust's terror is enhanced with the camera panning backwards, showing the door that led to their child's death and millions of others.