An iron mine is a placeable item that produces iron over time. It needs a power source such as a wind mill or copper engine.
You craft it with 15 iron, some wood blocks and some other materials. Sinces it needs iron to make iron, you can keep making iron mines faster the more you have.
There aren't just iron mines. Stone mines and copper mines and gold mines and silver mines and platinum mines are all the other mines. They produce the material it says in the name.