"My hands trained for battle, and my arms can bend a bow of bronze." This teaches that David would take a bow of bronze and bend it. Another interpretation is that it is easier for a person to bend a bow of bronze than to bend the arms of David. Another interpretation is that the bow of bronze came from the lineage of Nahshon. [edit: This is a quote from the book of Samuel (2 Samuel 22:35) that says "He teaches my hands to make war; so that a bow of bronze is bent by my arms."
The commentary explains that this teaches us that David was skilled in using a bronze bow and was able to bend it easily, indicating his strength and prowess in battle. Another interpretation suggests that it is easier for a person to bend a bronze bow than to subdue one's own impulses and desires, represented by David's metaphorical "arms." Finally, the Midrash teaches that the phrase "a bow of bronze" refers to David's ancestry, as he was descended from Nahshon, whose name means "bronze serpent."]