Mostly, you can delete the feeds comming from a specific site or from a category.
Also you can delete all feeds from your database[Empty database].
Orphan feeds is the feeds resides in database whom feed url was deleted [At the moment of a feed url deletion you have checked the "No, leave the feeds in DB" option, so the feed url was deleted but the feeds of this feed url was left in database].
If by any reason your are using "Empty database" option you must use "Refresh Feeds" option right away because at this moment there isn't any feeds in database, thus in homepage or marquee/scroll blocks. You can also refresh the feeds manually from "Feed URL Management" area.