It's a small utility that lives in your menu bar.
Reflex takes control of your media keys, and directs them to the app of your choosing, as long as that choice is Music (née iTunes) or Spotify.
It can also optionally take over your volume keys to make them control your music app too, if that's what you're into.
Because the media keys in macOS control whatever app is foremost, or was last. Maybe? It's confusing.
Imagine you're listening to music, but you just clicked a YouTube link, causing a video to start playing. Of course you want to pause your music, so you hit the play/pause key. But it doesn't pause your music - instead it pauses the video you're trying to watch. Frustration!
With Reflex running, the media keys are consistent and predictable, and always do what you want them to do.