Party mode
When several devices are playing from the same server, the app can show you all of them — and let you take the controls of any session from your own device.
Now playing on the server
The server's now playing overview lists every active playback session: who's playing what, on which device, and how far along they are. It updates live.

Remote-controlling another device
Pick a session from the overview and you get a full remote control for it: the same player view you know from Playback, but steering the other device. Play, pause, seek, skip through the queue — the music keeps coming out of the device it was playing on, while you drive from your phone.
This is what gives party mode its name: put a device on the speakers, and everyone at the party can queue up the next track from their own phone.