As they say, iCal. I think it's one-way, at least it used to be, and it's not real time. That is, your ResKey calendar will automatically update on Air a few times/day, but not instantly as changes are made. So it IS possible for a room to book, but still look available for a while on Air.
Air will show your ResKey availability, but reservations made via Air need to be manually entered in ResKey. Still a LOT better than keeping your availability calendar on Air updated manually.