Iron plate is Iron smelted in a Smelter, which leaves slag. Nearly every chain starts with it: iron plate goes into steel plate, gears, circuits and motors, and the Dock, Fabricator, Mixer and Refinery are built from it.
The Recycle Recipe also turns scrap back into iron plate, so the material of a scrapped facility can be recovered.
Iron is the largest share of the material in every tier.