The IGN video (via VG247) shows how it works. There's lots of obvious stuff, like being able to look at your characters, track stats and browse the Grimoire . There are also what Bungie are calling 'advisors' to help keep track of missions, bounties, special raids and what's available to buy in the Tower. Most useful is the ability to change loadouts and equip gear on the go.
It will eventually be available online as well as iOS and Android but for the beta it's iOS only (no word when it's actually out either so I'll keep you posted).