Iron is a raw resource used primarily in manufacturing Steel.
Iron Mines produce 0.25 Iron per turn each, accumulating to 3 Iron per day. Iron Mines are reasonably cheap to build and operate, and never run out of Iron to mine. Each city can have up to 10 Iron Mines.
There are no National Projects that increase Iron production. To increase Iron Production, a nation must increase the number of Iron Mines it has.
Iron, along with Coal, is a necessary component in producing Steel. Steel is a very valuable manufactured resource, and only nations in Europe and North America have access to both Iron and Coal. Nations in other Continents that wish to produce Steel must import either Iron or Coal, increasing their value as an export.