The title refers to the bow symbol used by Daniel, Joel, and Thacia in Chapter 7 that is also found in Psalms 18:34 in the Bible. Daniel has lived most of his life with hatred for the Romans. His hatred fuels his desire for revenge, and when an opportunity to get what he seeks presents itself through Rosh, he takes it and engages the Roman soldiers. 

Daniel narrates to his friends how the Romans killed his parents, leaving his sister traumatized and forcing his grandmother to sell him into slavery in order to make ends meet. Daniel succeeds in recruiting both Joel and Thacia in his fight against the Romans. They use the bow symbol to communicate secretly. Understandably, the idea to use the symbol comes from King David through the Psalms.  

Daniel and Joel take the symbolism literary, while Thacia asserts that the symbol demonstrates that it is God that gives people the ability to overcome their challenges. Daniel’s challenge is his hatred for the Romans, but through Jesus’ teachings he learns that love is a more powerful force that will “bend the bronze bow”.

Was it possible that only love could bend the bow of bronze? - Daniel, Chapter 24.

The title of the book refers to a passage in Psalms, the book of songs and verse attribute to King David. In the book, the passage is used to show Daniel's progression from revenge to love; he is unable to get past his own trauma, "bending a bow made of bronze," until he accepts love into his heart. Without love, his anger only pushes up against the immovable force of the Romans, threatening him with breaking instead of bending. In Chapter 7, Daniel decides to use the imagery of the bronze bow as the symbol of his resistance fighters:

"--God is my strong refuge,
and has made my way safe.
He made my feet like hinds' feet,
and set me secure on the heights.
He trains my hands for war,
so that my arms can bend a bow of bronze."
(Speare, The Bronze Bow, Google Books)

The Psalm is number 18, which details praise for God in times of trouble. The narrator describes how God infuses his mind and body with the power to overcome his enemies, and in particular to bend the bronze bow, which is harder than a wooden bow. In theory, a bronze bow would send a more powerful arrow; the symbolism is that a man must be strong enough, either through God or through self, to meet seemingly impossible challenges.