For filtering content, on the NRS thread Jim suggested a refinement of Josh’s idea above of using bookmarks, which would be that sites that people you are linked to had bookmarked would “bubble up to the top.” Hopefully wouldn’t throw up too many nasty surprises!
Essentially I think RDF can be seen as a mark-up language so that text can be easily sorted through by eg. search software. For example in the HTML for a site, the word “David” might be labelled as a person’s name (among other things.)
For it’s potential use in the search scenario I found the most obvious explanation on schema.org, but from what I understand schema itself is frowned upon as being much less sophisticated than, for example, SoLiD.
Apologies if I’m stating what you already know, or if I’m getting it wrong. @happybeing might be able to correct me if I’m misrepresenting it!