Archiving is a new feature (as of January 2019) that allows you to clean up your reservations list by hiding certain reservations from view. This is useful if someone has inquired multiple times, creating multiple entries.
In your Reservations list, you will see an icon of an eye with a line through it, just below the edit/pencil icon.
Click this icon to archive the entry.
Archived reservations are still easily accessible. Just click "Show All" and select the "Archived filter to view archived items.
Click the icon of the eye to un-archive a reservation. It will now appear with the rest of your reservations from the "Show All" view.