What was the relationship between Armand's mother and father

Armand's mother and father were deeply in love. They were married, and before her death, she thanks God for his love, but she is also thankful that God arranged their lives so that Armand would never know his heritage.

But it was not Désirée’s; it was part of an old letter from his mother to his father. He read it. She was thanking God for the blessing of her husband’s love:—
“But, above all,” she wrote, “night and day, I thank the good God for having so arranged our lives that our dear Armand will never know that his mother, who adores him, belongs to the race that is cursed with the brand of slavery.”