If you tend to want to keep your music running when you log out, or control your music playing on a desktop machine from a laptop, for example, you may well use the Music Player Daemon (MPD). If you use Ubuntu’s Unity desktop or KDE’s Plasma desktop, you may well wonder how to get the Ubuntu sound menu or Plasma’s Now Playing widget to talk to it.
Both of these use MPRIS2 to communicate with media players. MPD, however, does not have an MPRIS2 interface. Even if it did, it would take a bit of work to be able to use it from another computer. What you need, then, is a “bridge” of some sort to translate. One such possibility is mpDris2 (which, I hasten to point out, I haven’t tested).
You would run such a bridge on the computer you want to control MPD from, and point it to your MPD instance. And voilà! Every MPRIS2 client application that you run can talk to MPD, without knowing anything about the MPD protocol.
If you just want a headless music player on your local machine, though, you can always use Raven, which talks MPRIS2 natively.