Explore is Flickr's way of showcasing the most interesting photos within a given point in time -- usually over a 24 hours period.
Flickr receives about 6,000 uploads every minute -- That's about 8.6 million photos a day! From this huge group of images, Flickr chooses only 500 images to showcase for each 24-hour period.
That's only one image in every 17,000!
I have made Flickr Explore 14 times!