Signed up to test this a while ago when they announced it and just got accepted into it a few days ago.
Basically, connect your Xbox controller to your phone or tablet by Bluetooth and you can stream and play games anywhere. Not many games available atm but more to come when it launches. Pretty cool concept, could be a massive thing for people who travel a lot. Only had a few games on Tekken 7 with it so far but looking forward to trying it out some more.