The boy's mother is going off to sell cornmeal to get food for them after his father is taken away by the sheriff.  He stands at the door and watches her go away up the road with the meal sack over her shoulder.  The quote refers to the way things appear to get smaller and smaller as they get farther and farther away.  The boy stands there watching his mother walk away until all he can see of her is the sack, which stands out against the landscape the longest because of its whiteness. Soon even that small speck of white disappears from sight (Chapter 3).