The “Discover” feature is something new from the Google Reader team.
By clicking on the “Discover” link, Google Reader gives you recommendations for new feeds, which are generated by comparing your interests with the feeds of users similar to you.
In my case that worked pretty well, I got recommended feeds that matched the content of my other feeds.
Moreover, some recommendations were coming from Greek web sites, which matches some of my existing feeds and the location in my Google profile.
I am happy that I have subscribed to some of the recommended feeds, as I am always looking forward to new stuff.
Once again, well done to the Google Reader developer team.