The normal editor does not give you the opportunity to modify it openly. config and main function are the two basic functions every map is supposed to have. config contains information about the player setup, forces etc., is run in lobby. main is activated during loadscreen, does some default functionality like setting of camera borders, light environment, creates preplaced objects, starts the blizzard.j and finally initializes your triggers.
You can see both config and main in the editor by causing a syntax error, then scrolling to the bottom of the compile error window.
In JNGP, vJass offers
//! inject config
//! endinject
analogue for main
to integrate custom lines in those functions.