The narrator had just had a baby, and afterwards, had been feeling down and depressed. Today, we know this as post-partum depression; in her day, it was labelled as an unknown "temporary nervous depression." Her husband, a doctor, determines that she needs to get away from her busy life and rest. He chooses to rent out an old colonial mansion in the countryside. Supposedly, the removal from busy pressures and responsibilities of her old life, in combination with the fresh country air, will help the narrator to rest and recover.
The mansion was home to wealthy people at one point; she is put in the nursery, a room where children were tended by their nannies. She describes the torn wallpaper, the marks in the wood of the bed, and other signs of children having been in the room. The mansion is empty now because of "legal trouble...about the heirs and coheirs." So the people that were supposed to inherit the estate are having money problems, so it's been emptied and rented for cheap.
The narrator hates the place, especially the room she's been put into. She feels right off that there is "something wrong" with it; at first she thinks it is haunted, but dismisses it and tries to be happy with her circumstances. However, as time passes, her distaste for the house, and especially the wallpaper in her room, grows, then turns into a strange and unhealthy fascination.
I hope that those thoughts helped; good luck!
The Mansion adds to the Gothic sub genre and macabre subject matter, it sets a tone of the house almost being huanted, as the narrator states in the beginning of the story.
Hope this helps