Tell me, have you ever tried to read a book while running? If you have then you know it's very difficult to keep your eyes focused on the page (and then there's the whole issue of running into trees, cars, and other people).
The Run-n-Read can't fix the second problem, but is intended to solve part of the first problem. The data from its sensors is used to make the contents of your tablet's screen move around in sync with your head movements. The developers are hoping this will make it easier to read while running on a treadmill.
I'm not convinced it will work as well as they claim, but the slick promo video says otherwise:
The Run-n-Read also includes a way for you to tap it and turn the page, but that only works with the included app. BTW, the promo video suggests that this accessory won't work with any other app, so if you prefer a specific reading app then this might not be for you.
If you're interested, you can pre-order a Run-n-Read for $55. As for me, I am skeptical that the average Android tablet will be able to process the data fast enough to keep the screen motions in sync. I want to see a real user report before I beleive the claims.