The yellow wallpaper??

Pretty much what Raatz says. If you look at earlier in the story, she is not allowed to go outside, read, write, etc. She's just supposed to sit and get well. It was the idea of how fragile women were. That would drive anyone mad.

Her doctor husband locks her up because he thinks she's depressed/hysterical. The confinement drives her insane, she focuses on the wallpaper because it's the only mental stimulation she has. Once she descends into psychosis, she won't leave the room because she doesn't feel safe outside.