The woman behind the wallpaper is the symbolic representation of the narrator who is a prisoner of the domestic sphere of her life. Falling under the feme covert laws of the Victorian Age, the unnamed narrator--this anonymity also indicates the woman's lack of identity in her society--is denied any voice in how to effect her cure for her "nervous condition," which, in truth, is post-partum depression from childbirth, and the woman behind the wall gives the narrator an avenue of expression.
Constantly refused her requests such as being able to go into the garden of the summer house, having the unsymmetrical and horrible yellow wallpaper removed, and being able to read and write, the narrator becomes so repressed in her already unstable and lonely state that she suffers delusions of a woman who struggles behind the bars of the pattern of the wallpaper for her freedom. In addition, the repressed narrator has synesthetic perceptions; for example, the yellow wallpaper makes her think of
all the yellow things I ever saw, not beautiful ones...but old foul, bad yellow things....
There is a subtle, but "enduring odor" in the room. It is a smell that is "like the color of the paper! A yellow smell." This phenomena of synesthetic perception emits from the narrator's imprisoned mind that perceives the color as a pungent smell.
So trapped and frustrated does the narrator feel that she seeks to free the woman (Jennie) "creeping about behind that pattern," especially after her husband John begs her to never think about or mention her desire to leave the summer house before their rent has ended. Finally, symbolic of her self-assertion, the woman trapped behind the wallpaper is freed, but it is at tremendous cost to the patient, who herself creeps along,
...here I can creep smoothly on the floor, and my shoulder just fits that long smooch around the wall, so I cannot lose my way.
When her husband enters the room, the narrator tells him as she creeps along, "I've got out at last...in spite of you and Jane. And I've pulled off most of the paper, so you can't put me back!"