For every 100 "steps" made (you can obviously just game this by shaking the phone), you level up, eventually evolving your robot through all the generations that preceded ASIMO. As a bonus, for every 'bot evolution, you get to watch a movie of it in action. Once you unlock ASIMO himself, you can virtually tackle the Suzuka Circuit and compare your times against other players, as well as brag via the usual social networks. Once a million users complete the Suzuka level, the real ASIMO becomes sentient and embarks on his ultimate mission to annihilate humanity. (Kidding! We think.)
Want more details? Honda has them posted here. You can download Run With ASIMO for free in the App Store.