The smelter is a block that allows you to smelt together two ingots to create a new type of ingot. The smelter can only be used after the ore has been smelted into ingots. For now, it can only smelt copper and tin ingots together, to make bronze ingots.
The smelter can be crafted like so.
In case your browser doesn't let you see it, you arrange 8 cobblestone around a furnace.