What is item archiving in Rocket League?

Items that you do not want to see in your inventory can be added to the Archive tab. The Archive tab is found in your inventory and will only appear when there are items archived. You can search for items that are archived by selecting the Archived option from the Filters drop-down. Archived items can also be Favorited. You can remove an item from the Archive tab by clicking the Unarchive button. 

For more info please check out, How Do I Archive My Items in Rocket League?