The author uses words to draw a picture of isolation, depression, mental anguish. The story takes the main character to an isolated home away from their usual lives in order to let her "rest" and "recover".
The yellow wallpaper drives her crazy, literally, and she thinks she "sees" someone behind the paper "creeping" and "crawling." She ends up creeping and crawling along the floor herself.
So, look for descriptions that show the narrator has a mental disturbance or that other characters in the story think she has a mental disturbance.
She does keep talking about how "they" think she's "crazy". All of this lends to the tone and mood of the story and sets us up as readers for the ending.