Oprhan Stashes are stashes, that are no longer referenced by any scene object.
Removing Stashes from an object, or deleting the object itself, will not remove the previously stashed objects.
Instead they can still be viewed and retrieved using the View Orphan Stashes tool.
The View Orphan Stashes tool then is essentially the same as the ViewStashes tool, although a bit more limited.
No selection is required, only the presence of orphan stashes in the blend file.