The wallpaper is the screen onto which the narrator projects her fears. Its pattern makes her anxious about invisible supervision. At first, her condition is disquiet. Then it turns to obsessive anxiety, and, finally, madness. In the end, we witness an act of aggression.
Detailed answer:
Charlotte Gilman’s The Yellow Paper proves that suppression is destructive for a woman. The wallpaper in the room where the narrator is kept is critical to understand the author’s intention.
The woman sees a vague shapeless female silhouette. It looks mysterious and provocative for her and grows more visible every day. She discerns a woman, “stooping down and creeping about behind that pattern.” Over time, she discovers that the image on the wallpaper changes in a different light. At night, the external pattern “becomes bars!” With this discovery, the character’s attitude to the room changes. The passive prisoner becomes an active researcher.
In the daytime, this woman is restrained and quiet. At night, she begins to “shake violently” the bars and tries to free herself. Sometimes the narrator thinks that there are many women behind bars. They crawl quickly on the wallpaper. She associates “crawling” with a protest against women’s humiliation. Society limits their mental and physical freedom. The wallpaper pattern reinforces the room’s earlier association with the prison. In the end, the narrator tears the wallpaper off the walls. She proclaims freedom from supervision and liberation of mind.