Smelting is the act of refining raw stone ore(s) into useful bars of pure metal(s) or alloys. Smelting is similar but distinct from melting, which reclaims existing metal objects back into (fractional) bars. Metal bars produced at a smelter are usable in a wide variety of goods, from weapons to goblets to furniture to buildings.
Smelting also refines bituminous coal or lignite into coke, a fuel which is equivalent to charcoal, but first needs fuel to do this. Fuel is required to operate a smelter, metalsmith's forge, kiln, or glass furnace, but not their magma-powered equivalents.
Smelting requires the ore (or coal) to be processed, a smelter, either fuel or (for magma smelters) magma at least 4/7 deep, and a dwarf with the furnace operator skill enabled. Training in the skill is not necessarily required - since bars have no quality the only difference is the time the labor takes.
Since smelting coke requires fuel in a conventional (non-magma) smelter, usually one wood is burned in a wood furnace to create one charcoal to start the process. Alternatives are to bring one bar of charcoal or coke with the expedition at embark, trade for one from caravans, or go straight to magma smelters.