"So" primarily means "as," "like" or "such."
So ein schönes Lied!
Such a nice song.
Das tut uns [ja] so leid!
We feel [like] this [much] bad for [you].
So? Das wäre aber sonderbar.
Is that so [like this]? That would be strange.
Du darfst nehmen, so viel wie du willst.
You may take as much as you want.
Alles ging so weit (bis dahin) gut, aber dann...
All went well thus far, but then...
In addition, if used at the beginning of a sentence, followed by a comma, it means "well."
So, bist du schon fertig?
Well, are you done already?
HTH