What are resource items? Tutorial didn't mention anything about port maintenance. Help!

What are resource items? Tutorial didn't mention anything about port maintenance. Help!
That´s really simple. When you are in the screen where you can upgrade a building you can see the cost in gold and the time it takes to build and the new upkeep to pay after the upgrade. And there is a symbol of the item that is needed to be able to upgrade - e.g. 40 tons of Iron Wares that are used up in upgrading the building.
Any society that would give up a little liberty to gain a little security will deserve neither and lose both. -Benjamin Franklin
Hoi1 Resistance
Rappelez-vous quel Voltaire a dit; "Dieu n'est pas du côté des grands bataillons, mais est du côté de ceux qui tirent mieux"