Smelting in Minecraft is a method where you produce goods in game. Also you can call this Cooking, Burning, Baking etc. This is same like crafting but there you put ingredients and corresponding output which you will get. Smelting requires furnace blocks, you can find also heating option there and choose which object will be heated and how much. Also for furnace box you need fuel.
How to smelting? Add material and fuel in furnace and it will automatically begin smelt and consume one piece of fuel. Fuel is many cases are coal. If your smelting process are long then furnace will use another piece of fuel (coal). If you don’t have enough fuel process will be stopped and you need to put new fuel in furnace. How to figure out how much time will take smelting? You can find arrow icon and watch cooking, every operation take ten seconds so do your math.
You cant put something and smelt it and go outside, when you do this operation you must be near furnace. If you sleep in your bed, when you wake up smelting process will be at same point. Here you can see some recipes for smelting.