What happened in chapter 10 of The Boy the Striped Pajamas?

In Chapter 10 of The Boy in the Striped Pajamas, titled The Secret, Bruno goes on a final secret exploration of the camp and sneaks out of his father’s house to visit Shmuel. He explains that he is moving tomorrow and Shmuel is devastated. At the camp, he sees children being separated from their mothers, and when Bruno reaches the fence, he sees a large group of people huddled together and guarded. He and Shmuel share some bread, and Bruno remarks on how cold it is. Then he notices that Shmuel looks different in some inexplicable way. As he turns to get a closer look, he sees that he is wearing striped pajamas.