It's a concept about interactive and keyword based navigation. Search a given keyword will give you photos. Click on a picture will give new keyword... and etc.
Click a word to see related images.
Click an image to see its tags.
Click an image's title to go to its Flickr page.
Click the background to go back up the chain.
Drag to pan, scroll-wheel to zoom.
Or just hit the spacebar and enjoy.
It's nice and well done.