The Furnace is unlocked once you reach the Cave Biome.
Once you have unlocked it, you can navigate to the Furnace menu with the Backpack icon on the lower left.
You can use this to get rid of the excess resources you have and turn them into the much more needed rare blocks!
To unlock the x50, x100, and xMax options, you need to unlock the skill in the Skill Tree: Idle - 'Durable Furnace'.
The Boost Furnace button allows you to speed up the furnace for a period of time, by watching a short video.