tumblr is a huge website where users can register their own websites (subdomains) and add content. So for example funnyCats.tumblr.com. This new tumblr sub domain (website) might become very popluar and attract a large amount of traffic and organic whitehat backlinks. However if this user no longer decides they want this tumblr website, after a unspecified amount of time with no tumblr account logins tumbr will expire it. Once expired its free for anyone to reregister. Now you have a tumblr website to do with as you please.