Its for information exchange. If there a great thread and or article and you wrote it or the information is worth spreading, you can "digg" it. Digg.com is a collection of the latest articels/information spread out by people for information exchange. If you don't use the site, you can simply ignore it. Its a good way of finding the latest articles from 1000''s of websites that people around the web submit. Concolidates everything in a way...
"Digg is all about user powered content. Every article on digg is submitted and voted on by the digg community. Share, discover, bookmark, and promote the news that's important to you! "