Symbolism, Imagery, Allegory
Mrs. Thornton doesn't talk much about her dead husband. After all, the guy pretty much plunged her family into poverty with his drinking habit. But Mrs. Thornton still cherishes the embroidered napkins that she used on her wedding day. When she realizes that her son John is going to propose to Margaret Hale, she starts to pick off the embroidered initials in order to replace them with new ones.
As the book tells us, "Mrs. Thornton stood looking at them long—they had been her pride when she was first married. Then she knit her brows, and pinched and compressed her lips tight, and carefully unpicked the G.H. She went so far as to reach for the Turkey-red marking-thread to put in the new initials" (2.1.5).
Mrs. Thornton isn't happy to be doing this. She doesn't like Margaret and is heartbroken to be losing her son to another woman. Little does she know that her work will be in vain, since Margaret ends up turning John down (at least the first time). We can tell from the seriousness of the description that Mrs. Thornton's attempt to replace the initials on her wedding napkins is a symbolic passing of the torch to the younger generation.
It's also symbolic of Mrs. Thornton's jealousy toward Margaret. Not jealous-jealous, because she's not a weird reverse-Oedipus that's in love with her son or anything creepy like that. But she does feel as though she's being replaced. Her preoccupation with her wedding napkins shows that she feels as though she's both losing her son and losing her own identity as a bride.
This activity is also symbolic of Mrs. Thornton's tendency to totally jump the gun. Remember that she's preparing to embroider Margaret's name on these napkins before she even knows if Margaret has accepted John's proposal. But Mrs. Thornton is just like that. "Mrs. Thornton was not a woman much given to reasoning; her quick judgment and firm resolution served her in good stead of any long arguments and discussions with herself" (1.12.10).
Yep. We have "quick judgment" and "firm resolution" coming out in spades here. She's doing the embroidery equivalent of counting her chickens before they hatch.