In Harper Lee’s novel To Kill a Mockingbird, Mrs. Henry Lafayette Dubose is the Finch family’s neighbor. She is also virulently racist and repeatedly demeaning to Jem and Scout Finch. She is an elderly woman who comes across as unfailingly angry and bitter, and, one day, Jem has finally had enough of her verbal abuse directed against him and his family. In a fit of anger, he destroys Mrs. Dubose’s prized flowers, camellias. As Scout, the story’s narrator, describes the scene, “He [Jem] did not begin to calm down until he had cut the tops off every camellia bush Mrs. Dubose owned, until the ground was littered with green buds and leaves.” Sent by his father to apologize for his actions, Jem’s returns home and informs Atticus that Mrs. Dubose wants him to come over read to her every afternoon after school, and on Saturdays, too – a punishment with which Atticus is in agreement.
It is only after Mrs. Dubose dies later in the novel that Atticus provides his children the old lady’s story and a reason for her orneriness. Mrs. Dubose had been seriously ill for some time, and became addicted to the morphine used to treat her pain. Atticus explains the recently deceased old lady’s malady as follows:
“Mrs. Dubose was a morphine addict,” said Atticus. “She took it as a pain-killer for years. The doctor put her on it. She’d have spent the rest of her life on it and died without so much agony, but she was too contrary—”
Mrs. Dubose, Atticus reveals, has left something for Jem: a single white camellia.
The answer to the question – what did Jem have to do for Mrs. Dubose, and why – is he had to spend afternoons reading to her for a month because he destroyed her flowers.