Bronze is a manufactured metal that does not occur naturally. Bronze bars can be used to make reasonable weapons and armor that are 75% effective compared to iron. Bronze can also be used to make furniture and other objects at a metalsmith's forge.
Bronze must be smelted by combining malachite (copper ore) and cassiterite (tin ore) - you cannot combine existing bars of copper and tin to do this. It is more valuable than either copper or tin and is the strongest material available before the discovery of iron (except for obsidian short swords).
Producing bronze is also more efficient than producing most metals, as you only need one unit of fuel to turn one malachite and one cassiterite into two bronze bars, allowing you to produce more goods without needing as much fuel (at least until you have a magma smelter).