You may have seen an "Hide" selection in the bulk actions drop down menu. If you have items that you don't plan on listing, and don't want crowding up your view, you can check that item as "Hide" and it will be hidden from view on your Amazon Inventory tab. This is a tool to make managing your inventory a little more convenient.
If you want archived items to show up again, simply select "Hidden" from the Archived filter on the Amazon Inventory Tab to see items you have hidden. Select the item(s) you wish to unhide from the drop down menu and select "Unhide" and then "Apply Action" to have the item re-appear with your other non-hidden listings.