This achievement will unlock once you have smelted and retrieved an iron ingot. This must be done in survival mode.
This is done by putting iron ore and fuel into a furnace, waiting for the process to complete, then taking the iron ingot and placing it in your inventory.
Iron ore can be found while mining, it is often in cave systems and looks like stone but with brown bits in it. Coal can be found while mining as well, it looks like stone but with black bits in it. Other fuel can be used such as charcoal and wood. Charcoal is created by burning wood in the furnace.