Juliek is a fellow prisoner in Elie's camp. He is a talented violinist who is clearly passionate about his music. When everyone else is simply focused on survival when the camp is evacuated and the Jews are forced to march to a second location, Juliek manages not only to survive, but to keep his intact violin with him.
As he is dying, Juliek uses that violin to play a song. Specifically, he plays Beethoven, an act that he was not permitted to do in the camp. As Beethoven was a famous German composer, Jews were not permitted to play his songs. Playing Beethoven was not only an act of defiance for Juliek, it likely also comforted him in this incredibly difficult time.