This is a great reference on what Flickr Explore is, what Interestingness is, and how to understand both:
What is Explore?
Explore is a Flickr feature with the intent of showing you “some of the most awesome photos on Flickr.” Photos are automatically selected by computer according to a secret algorithm called Interestingness (see below for more about that).
Is Explore a showcase for the top Flickr photographers?
No. It’s for photo viewers, not the photographers. It exists so that, at any moment, anyone who wants to view interesting photos can go to Explore and have a reasonable chance of seeing something interesting. Does that imply that photographs not in Explore are uninteresting? Of course not. There are many, many wonderful photos uploaded to Flickr each day that aren’t selected for Explore. But to serve its purpose, Explore only has to include a small sampling of all of the photos on Flickr (currently at 500 per day or about 0.005% of the daily upload volume). And Explore tries to show photos from as many different people as possible to create a diverse selection.
Explore is for the viewers. It’s a way for Flickr to show the world a sampling of what is being shared there. It’s there for those who are new to Flickr, who are lost in the vastness of it all and don’t know where to begin. It is not a “best of” listing of photographers. It is not a popularity contest.
Interestingness is what Flickr calls the criteria used for selecting which photos are shown in Explore. All photos are given an Interestingness “score” that can also be used to sort any image search on Flickr. The top 500 photos ranked by Interestingness are shown in Explore. Interestingness rankings are calculated automatically by a secret computer algorithm. The algorithm is often referred to by name as the Interestingness algorithm. Although the algorithm is secret, Flickr has stated that many factors go into calculating Interestingness including: a photo’s tags, how many groups the photo is in, views, favorites, where click-throughs are coming from, who comments on a photo and when, and more. The velocity of any of those components is a key factor. For example, getting 20 comments in an hour counts much higher than getting 20 comments in a week.
(continued via Frequently Asked Questions / Scout)