Please help explain this long sentence
Solution 1:
Peter Shor is right, this is from heraldry.
sable means the color black
gules means the color red
The field in question is just the background.
What it means is that Hester Prynne was required, because of her adulterous ways, to wear a scarlet letter A (for adultery or adultress) with a black background (which may have been her dress, I don't remember).