The Yellow Wallpaper explores the theme of how 19th century society restricts and infantilizes women, particularly in the context of mental illness. Two key details that show this theme are:
1) The main character's husband John, who is also her physician, insists she is simply suffering from "temporary nervous depression - a slight hysterical tendency" and forbids her to work, write, or pursue intellectual activities. He confines her to the nursery and prescribes "rest cures" like keeping her in a secluded mansion and forbidding mental stimulation. This reflects the patriarchal view of the time that women's activities should be limited to domestic and nurturing roles, and that intellect and independence in women was abnormal.
2) The main character is not taken seriously when she tries to explain her mental condition. When she expresses her state of mind to John, he dismisses her worries as "a small thing partly inherited, and partly due to overwork." This shows that even women's own descriptions of their inner states were not trusted or valued, reflecting the belief that women were irrational and prone to hysteria.
So in summary, women are viewed as irrational, emotionally unstable, and only suited for domestic duties. Active intellect and creativity in women is seen as abnormal and something that must be suppressed through restrictive "treatment."