Smithing is a crafting skill. With it, you can turn metal ingots into many useful items. In order to make a living as a smith, your character will likely need to have access to a hammer and anvil, and a source of heat to soften the metal that is being forged.
Smithing is difficult to learn on one's own, as it requires a steady supply of raw materials or money with which to purchase them. A character interested in becoming a smith is advised to become an apprentice to another character who is already an accomplished smith.