What is PubSub?
"PubSub" is short for "publish and subscribe". It refers to a centralized information-processing system in which material fed into the system from a variety of sources (that's the "publish" part) is sorted, matched, reformatted, and sent along to "subscribers" according to their particular interests. The heart of the system is our blazingly fast Internet-scale "matching engine", which allows us to process new information and get it out to all of our users as soon as it appears on the Web.
They go to great pains to distinguish themselves from search engines. Search engines crawl the web and make an indexed collection of what's on the web. They call themselves a "matching engine". So, what the heck is a matching engine? What they say is "we maintain a collection of our users' queries, and match them in real time against new documents as they appear on the Web".
For this to work they have to know about your RSS feed. So, to add your feed go to their site and click on the "Tools" link in the navigation bar.