When a blogger publishes a post and other bloggers link to it, the post becomes part of a conversation. The initial post is the "seed" of the conversation. What happens next is fascinating. Posts linking to the seed are themselves linked to, and so on. From the seed post grows a conversation graph. The nodes of the graph are posts and the arcs of the graph are permalink citations from post to post.
The graph makes things a little more interesting that just viewing the regular trackback function offered through services like Movable Type and used on many blogs. For the stats fetishists, it's just another set of numbers to obsess over.