Well, Joseph could better answer the question because he uses it for this site (and I don’t). But basically, if you have an RSS reader, you can subscribe to blogs and the like and be notified when they update.
If you click the link “RSS feed for this topic” (at the bottom of every thread above the reply box) you will see the feed – XML code that your reader will translate for your reading pleasure. When a thread is updated, YW releases a new XML and your reader downloads it and can notify you.