The power of imagination is seen as a vehicle for redemption, even as it is a vehicle for harm. Cecilia calls Briony "fanciful" as a way of saying she makes up stories and willfully presents them as truth, as she did with accusing Robbie of raping Lola. Briony's imagination is responsible for the novel's central conflict: the separation of Robbie and Cecilia for reasons that have nothing to do with anything they themselves have done. Briony's claim that she saw something she didn't (Robbie attacking Lola) is what forces Robbie to go to war, in order to be able to leave prison, and causes Cecilia to refuse to engage with her in any but the most cursory way.
Briony imagines that the two lovers do manage to meet up again, as a way of atoning for her actions. Interestingly, her imagining of their intimacy mirrors the scene she stumbled upon in the library, when they were making love, but which Briony in her youth and inexperience saw as a violent act. This in turn fed into the fantasy she created of Robbie raping Lola, because she did not want to think of Robbie as anything other than a "sex maniac" because it would mean he preferred Cecilia to her, and it was Robbie's scolding of her that made Briony, harboring a schoolgirl crush, feel rejected.
As Briony matures, she comes to realize how wrong her actions were, and also that her sister can never forgive her. She writes the novel as atonement for her actions, using the act of imagination once again in an attempt to redeem the pain she has lived through, and the pain she has caused others.