The yellow wallpaper symbolizes the protagonist’s deteriorating mental state, as well as her confinement and isolation.
The protagonist is confined to the room that she is in, and she is isolated from her family. She is also isolated from society at large because she has been essentially imprisoned by her husband. When the protagonist looks at the yellow wallpaper, it reminds her of how trapped she feels by both her husband and his expectations of how a woman should behave. It also symbolizes her feelings of entrapment and hopelessness: she knows there is no way out of this situation, so why bother trying? The yellow wallpaper could also symbolize the protagonist’s own repressed emotions and desires: she may be feeling frustrated with herself for being unable to cope with life in general (similar to how we might feel when we want to do something but just can’t seem to get ourselves motivated enough). The wallpaper could also symbolize the protagonist’s fear of going insane: because she has been isolated for so long and seen no one except for her husband who visits briefly every day or two, she may be afraid of losing touch with reality altogether—and if you believe that madness is defined by lack of contact with other people.