The quote from Ian McEwan's "Atonement" refers to a hidden aspect of the protagonist's life, symbolized by the squirrel's skull under her bed. It suggests a layer of darkness or a secret that is integral to her character, one that others are oblivious to or choose to ignore. This detail creates a sense of intrigue as it raises questions about the nature of what lies beneath the surface of people's lives.

Furthermore, the mention of the skull can be interpreted as a commentary on the themes of guilt and innocence that run throughout the novel. It emphasizes how certain burdens or truths may be concealed from others, reflecting the complexities of the human experience and the secrets we carry. Ultimately, this phrase serves to deepen the reader's understanding of the protagonist's inner turmoil while also hinting at a broader commentary on the hidden aspects of life that people often choose to overlook.