In the second sentence, the clean laundry on the clothesline are drying in the "gritty city air." This could be a foreshadowing of what is to come later in the poem when the children will be gone and the mother is left to herself in the old home.
The playclothes are no longer playclothes, but instead simply clothes. The laundry has lost some meaning because things have changed. They are not going to be worn by her children anymore, but she is cleaning them anyway.
The house has grown large and absent. Without children around to take care of, everything seems much more open, still, and vacant. The house is empty, nothing taking up space. There is seemingly more space than there was before.
As a new wind comes, everything is moving -- a metaphor for how the family is moving on. The narrator's son's blue shirt is pulled free from the line -- another metaphor of her son growing up and leaving.
The setting has turned to evening, symbolizing that it is at the end of the children's childhood. It is a dream, and as many dreams can be surreal, so was her experience with a small house and a small town.
The land is flat and spread out far beyond us. Everyone is free to go and explore the world, and it is a constant reminder for the mother as she gazes into the endless prairie.
"Don't I know you?" She remembers what it is like to be independent without her children around. The cooling light is another metaphor for the beginning of a pleasent new day and a new chapter in the mother's life.