This lists photo files sitting in the item photo directory that aren't linked to any item in the database — usually left behind by a deleted item or an interrupted upload.
Check individual photos and select Delete selected to remove just those, or use Delete all to clear the whole list. If nothing is listed, there are currently no unlinked photos.
These files aren't referenced by anything in IMPS, so removing them here doesn't affect any items. Deleting them cannot be undone.