The phrase "While Claude sets up the bookshelves" is a fragment. The reason that this phrase is a fragment is because a sentence is a set of words that is complete in itself. On the other hand, a fragment is a small part broken or separated off something. You might think of a fragment as a broken piece of pottery. This is because the broken piece had fallen off, therefore, making it incomplete. And this phrase is an incomplete sentence, making it a fragment.