This is the child skill of mining and repair. This skill benefits mining ores and smelting ores within a furnace. levels in smelting are gained by levelling up mining and repair. The total level of smelting is taking the levels of both repair and mining then divide it by two to get the smelting level.
Perks of levelling up Smelting:
Fuel Efficiency: Makes fuel more efficient. It is basically a multiplier that increases the time fuel is used for by 0.04 per smelting level up to 4x at 1000 smelting.
Second Smelt: Grants a chance to get two ingots for the price of one ore. This is increased by .1% chance per level of smelting up to a max of 100% at 1000 smelting.
Vanilla XP boost: Passive ability that grants extra xp from smelting things like iron ore. This basically makes stacks of vanilla xp more efficient whenever something is smelted.
Flux Mining: Unlocked at 250 smelting allows for a player to turn a pickaxe into a flux pickaxe which grants the ability to smelt an ore into an ingot at a rate of 1/3 chance You get a flux pickaxe by putting it into a furnace and it will turn into a flux pickaxe. In general, I recommend not using a flux pickaxe as you get more material overall if you don't use flux pickaxe.