At the bottom of each page of threads you see now a field named 'Tags'. There you can put some keywords (I think about 5?) that discribe the nature or contents of the thread, so that it is more easily found doing a forum search. That is at least my interpretation.
Edit: OK, it's a bit different. I just clicked the Tags line and it opens up a specific tag based search engine. The thread author seems to be the only one who can add Tags.