RDF is the Resource Description Framework. In essence, it is all about data about data (metadata). It is the basis for RSS, which is used by Slash; an RSS file does not normally give you the content of an article, but tells you the title, who wrote it, what kind of story it is, when it was published, etc. It is all a big step toward Tim Berners-Lee's Semantic Web
RSS 1.0, used by the next version of Slash, makes even more significant use of RDF than did the original RSS, so it is something you may be interested in understanding a bit better. Tim Bray, the XML guru, has written What is RDF? over at XML.com, and it is a good read.