Music production is a generic term to describe several steps of audio treatment in the elaboration of a track or a song.
The music producers are the musical and technical supervisors. They are the commanders in chief. They decide on what to do or not. They give their artistic touch on the track/song/album. They are also very often musicians, for it is very convenient when you have an idea to be able to record and work on it.
They usually assist to every phase of the process which means recording, editing, mixing and mastering. On the contrary it does not mean they necessarily realise everything technically speaking. They give their recommandations, wishes, orders.