Iron is a resource which must be gathered by peons in order to make Iron warriors. Iron is the second most plentiful resource which makes great for mass producing iron warriors. To gather iron you must have peons peasant in the Armory and then go to the "Gather Resource" section of your Armory. Click the "+" symbol next to the picture of a black rock. Another way to gather this resource is to select idle peons and then click on nearby iron and they will automatically start gathering the resource and bringing it to the armory.
Iron is also needed to make Chicken Warriors.