In "The Yellow Wallpaper," we learn about a woman who is being "treated" for a particular illness. We do not know what this is, although it is most likely a form of depression. However, the treatment she is being given is that of resting in a room with no mental stimulation.
This isolation causes the narrator to become obsessed with the wallpaper. What we learn is that the character has an intelligent, creative mind that needs to be stimulated. By depriving her of this, she slowly becomes more and more obsessed with the wallpaper, worsening her condition.