Spiritualism refers to the belief that the dead remain conscious and present in the afterlife and that it is possible for the living to communicate with them. This communication was customarily done through a medium during a "seance." A medium is a person who claims to have a special ability for communicating with the dead. Spiritualism is not nearly as popular as it was in Mrs. Kirby's time, but there are still plenty of spiritualists practicing. Some of them are complete charlatans, like so many so-called "psychics," but others sincerely believe in their special gifts.
Sir Arthur Conan Doyle, creator of Sherlock Holmes, was a firm believer in spiritualism and frequently tried to communicate with his dead son in seances. Doyle's short story "How It Happened" is supposedly dictated to a medium by a man who was killed in an auto accident. The story's ending is pure spiritualism.
Please refer to the eNotes link below for a discussion of spiritualism.