Chris recently had one of his photos show up on Flickr Explore, and he got over 49,000 views in one day. It's a search algorithm that has a few conditions - 1) It has to have a lot of views, 2) the image has to be of a minimum size or above 3) Pro users are more likely to get highlighted 4) The more active you are on Flickr, the more likely your image will get chosen. On the other hand, the more groups your photo is featured in, the less likely it'll be featured. 5) Avoid Watermarks.
If you get a picture highlighted on Explore, you will be prevented from getting another one on for a week to ten days. Ironically, the one featured that isn't really considered is quality. So don't get obsessed with getting onto Flickr Explorer. It's just a measure of community activity.