What's a producer?

What's a music producer really doing? Here is the answer with a concrete example. From demo to final result...

Coffee & Dreams //
Written by Camillefois ( http://www.facebook.com/camillefoismusic )
Produced by Arker

Mixed by Guido Aalbers
Mastered by Pier-Durk Hogeterp
Vocals recorded by Dirk Christiaens

Check out "Coffee & Dreams" official video: https://youtu.be/xHpAUinj4KU