Guest Passes let you share your photos that aren't public.
Anyone can see your public photos anytime, whether they're a Flickr
member or not. But! If you want to share photos marked as friends,
family or private, use a Guest Pass.
If you're sharing photos from a set, you can create a Guest Pass that
includes any of your photos marked as friends, family, or private. If
you're sharing your entire photostream, you can create a Guest Pass
that includes photos marked as friends or family (but not your private
photos).