What this means in the real world is that it is an Atom Publishing Protocol client.
Joe Friend’s latest post on the topic (Word Blog HTML Quality) states that
We will hand off valid XHTML for each post and
Clean HTML is more important than visual fidelity.
This means that if your Content Management System supports the Atom API, editors should be able to use Word to edit content.
Of course, the proof of the pudding is in the eating, but, given that Word is the first or second most used tool for writing text in the business world (the other being Outlook) this seems extremely positive to me.