It was supposed to be Copilot, with access to your Xbox account. So you could ask it things like "what achievements do I have in [game]?" It would also know what game you were playing, and answer questions like, "I'm stuck on [puzzle], give me a hint," etc.

The "on mobile" part was that it would be integrated into the Xbox mobile app.

(I say "supposed to be" because it was just a beta, and hadn't even made it to consoles yet – just PC and mobile.)