Same app experience across all devices ?
Means youtube app on xbox can run on Windows 10 pc's?
It's more than that, it also a sync of your data across devices.
So it would be opening up a youtube video in the same place you paused it on another device for example. Another would be editing a document - say you're working on a document on your phone and when you open word on the PC/laptop, you have the option to continue off that document at the exactly same place you stop editting on your phone. This does work but it means you have to save your document on onedrive, it is hit and miss due to the fact cellular / mobile data coverage is not exactly 100%.