IGN:
What did Joss and J.J. do before Star Trek and The Avengers?
Just what is a showrunner? It’s a term that many of us will have heard, but may not have a comprehensive understanding of. Now more than ever showrunners and television series creators are becoming celebrities in their own right.
Some, most notably J.J. Abrams and Joss Whedon, have gone on to balance their work on television with careers as feature film directors.
Nick Hershey of Entertainment Buddha writes: "Showrunners: The Art of Running a TV Show by Tara Bennett dives into the world of showrunning, covering how the title has changed over the past few years with the rise of social media and heightened storytelling. The book includes interviews with showrunners such as Joss Wheden (Buffy the Vampire Slayer, Firefly, Agents of S.H.I.E.L.D.), Damon Lindelof (Lost, The Leftovers), Kurt Sutter (Sons of Anarchy), J.J Abrams (Alias) and many, many more."
So basically show runners would be a manager at a store? You'd have the owner (executives) then Manager (Show runner) then employees (actors)
Only difference is the show runner is somewhat trying to fight for freedom and see what they can get, and not so much "making sure it runs"