What this means, essentially, is that you no longer need to wait for your news reader to ping your blog every so often to find out if there are any updates - you'll find out in real time.
PubSubHubbub, also referred to as PuSH, is a decentralized real-time Web protocol that delivers data to subscribers the moment it becomes available. Traditionally, an RSS reader would poll a blog every so many minutes, like an annoying child on a car trip asking if you're there yet. With a PuSH enabled blog, the blog and the reader both communicate through a hub. When new content is published, the blog immediately notifies the hub, which then notifies all of the subscribers. There is little to no delay. As Wordpress notes in its blog, "In most cases these updates are sent out with in a second or two of when you hit the publish button."
Just like the adoption of RSSCloud last fall, there is no need to opt-in or install a plugin for a blog hosted on Wordpress.com to become PuSH enabled - it's already active. For Wordpress blogs hosted separately, a PuSH plugin, PuSHPress is now available for download.
This is yet another big step in our progression to a real-time Web. Last month, Google Reader went real time by consuming PuSH feeds, meaning they show up on the news site almost immediately after being published to the originating site. In conjunction, this means that any Wordpress.com hosted blogs, as well as any PuSH enabled blogs running Wordpress, will be immediately available on Google Reader and any other reader set to work with PuSH.
This also means that, if you want to be on the razors edge of what's happening on the Web, you can also receive chat notifications of PuSH enabled blogs. RSS readers can be so last year when you can get a chat notification the instant a piece of content is published.