Traditionally services that process feeds (RSS & Atom) needed to regularly request the feed to see if there were any new entries. To reduce the need for constant polling rssCloud was introduced, as a way to ask for notifications when a new entry was published to a feed.
When you publish a new post WordPress.com will gather up the list of subscribers that have requested notifications via rssCloud and send them a small piece of data that lets them know that there is a new entry in the feed. It is still up to these third parties to process the notification and request a copy of the feed in order to actually get the content of the new entry.
<cloud> tag is included with the contents of the feed to let others know how to subscribe to the notifications. Services can then use this information to make subscription requests. Details on exactly how this is done are available at the rssCloud.org ‘requesting notifications’ walkthrough page.
Here is what it looks like in the feed for http://en.blog.wordpress.com/
<cloud domain='en.blog.wordpress.com' port='80' path='/?rsscloud=notify' registerProcedure='' protocol='http-post' />
Q: How do I enable this for my WordPress.com blog?
A: It already is! Every site hosted on WordPress.com includes supports for rssCloud.
Q: How long does it take from the time I hit the publish button until the notifications get sent out?
A: Our observations have been that they go out with in a second or two.
Q: Where can I learn more about rssCloud?
A: Check out rsscloud.org.
See also PubSubHubbub