It's part of the container tabs concept, but automatically putting each first-level origin (what you see in the URL bar) into its own container. This is atm only an about:config option in Nightly.
So how do you use this with systems that need it - can you log into the site for this "container"? Or would it leave the container when you go to the sign-in page? Like if I'm using Google or Facebook for single-sign-on, or I want to comment on a Disqus thread below a news article - since it's in its own container, I'm not logged in on this other site, but if I click a link to log in and it takes me away from this site, it takes me away from this container.
That's actually an interesting issue I had with it, in some versions that actually didn't work. But, somehow, now it does, and I don't know why (haven't read up on it yet).
