Seems like Flickr has added some new features. One of them is a way to share also private photos with other people who are not necessarily members of Flickr and therefor can’t be added to your friendslist etc.
The whole beast is URL based, so whoever gets this URL has access to these private photos. But this isn’t really a problem IMO since you can always expire those so-called “Guest Passes”.
This feature could really come in handy for me every time there is some family party and people want to get the photos I made but are too lazy to register and account on Flickr ;-)