Bruno, being the son of the Commandant of the Nazi army who is in control of the concentration camp that housed the Jewish prisoners, he is a healthy boy. On the other hand, Shmuel was sickly and thin, being a prisoner of the camp.
When Shmuel was taken int the house of the Commandant to help in the kitchen work. It was there that Bruno saw Shmuel for the first time outside of the fence. He had been taken "to polish the glasses [.... for] they needed someone  with tiny fingers". And once Shmuel held out his hand to show how "tiny" his fingers are, Bruno noticed how different their hands looked. His "appeared healthy and full of life" while "Shmuel's hand, however, told  a very different story".
Explanation: someone else already answered this question,  but I'm giving you the answer that was already provided