On Civ+, a 火 is a unbreakable Flint and Steel. When you left click with a 火, it creates a click sound and some sparks. In a crafting table, it can be used to smelt items just like a normal flint and steel, abet with unlimited uses.
To craft a 火, you use Flint and Steel and 24 stacks of Coal Blocks in a 5x5 Crafting Table.
The 火 is added by MeansOfProduction.