The narrator is upset about John’s choice of bedroom for her. The narrator prefers a lovely room downstairs that has nice decorations and a window overlooking the garden. However, John argues that the room is too small because it cannot fit two separate beds. He selects instead the nursery room (as indicated by the bars on the windows for children). A big room, the nursery has windows on all sides and allows plenty of sunshine.
Notably, the narrator wanted the more stereotypically feminine room, one that "opened on the piazza," with "roses all over the window, and such pretty old-fashioned chintz hangings!" Despite the airiness of her shared room with John, the barred windows symbolize her imprisonment. That the room may have been a former nursery is more important; she is forced into a helpless, infantile position with John as her caretaker.