It's a "radically redesigned game engine" that they've been working on, letting them render "seamless worlds of near infinite possibility." It's what lets us witness the "birth of a new nation".
The Ubisoft tech team are super proud of their know-how, which we'll get to judge for ourselves when Assassin's Creed III releases on Xbox 360 and PS3 this October, and later on PC.
If listening to PR hype isn't your thing then at least consider all the extra eye candy you'll be seeing of the next Assassin's Creed if you check out the AnvilNext trailer below. Up to "2000 non-player characters" can be rendered on screen "in a single sequence" giving us those intense looking battles of the American Revolution.