Please help explain this long sentence

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).

"sable" means black and "gules" means dark red or scarlet... so the contrast between the A and the rest of the world displays that the scarlet letter will always stand out and be a part of Hester, even after she is gone.