From the help file About backing up or deleting items using AutoArchive Expired items are mail and meeting items whose content is no longer valid after a certain date, such as a meeting you had four months ago that still appears on your calendar. Although an expiration date is optional, you can define it at the time you create the item or at a later date. When the item expires, it's unavailable and has a strike-out mark through it. You can set an expiry date for an email, right click click options, check the box expires after, then fill in a date. By default no date is set. So the option you ask about will delete any email messages that are past their expiry date. If you never set an expiry date on any messages, there won't be any to be deleted.