I'll let the video speak for itself.
Basically, it's an extremely modular controller meant to allow disabled people to configure something specific to their needs.
I really, truly think this is the coolest thing to come from Xbox. Gaming is notoriously bad for accessibility features. Fully customizable control schemes and even basic color blind options are still fairly rare. It was a fantastic moment when Sony allowed you to easily remap the PS4 controller at the system level, and this is on an entirely different level from that.
I can even foresee some cool configurations that people will come up with for things like speedrunning or attempting to get something similar to the old Steel Batallion controller.
I'm seriously considering buying one even though I don't need it, because goddamn is this just the best and I want to support it.
EDIT: Here's a pretty good rundown from Ars Technica that gets into some of the devices this is designed to work with: https://arstechnica.com/gaming/2018/05/xbox-adaptive-controller-a-bold-answer-to-the-tricky-world-of-accessible-gaming/