In Our Town, who is the hero and why when the Stage Manager says: "The real hero isn't on the stage at all, and you know who that is."?

This is
spoken in the context of a wedding, in which the Stage Manager (who plays all kinds of bit parts
in the play) acts as the minister.  Emily and George are about to begin their lives together;
and, despite their last-minute jitters, they are in love.

This line is a
clear reference to God, the one who ordained the rite of marriage.  He makes this reference
before the actual ceremony begins, and he's talking to the audience (us), not the guests or the
happy couple.  A line or two later he says "child born into this world" and
"perfect human being."  Coupled with his role as minister and his colloquial (local,
conversational) language, it's pretty clear he means God is the real hero of every
wedding. 

Remember, the Stage Manager's primary role is that of philosopher. 
He reflects outloud on the things in life that matter--which is the essence of this play.  He
has spent some time on birth, he will spend some time on death, and here we are in the middle,
which is love and marriage--and God.