In literature, a "hook" is the part of a story that grabs the reader's attention. Usually found in a book's opening scene, the hook serves to pull the reader into the world of the novel and keep them reading, as they want to know what's going to happen next.
Neil Gaiman's The Graveyard Book starts not by introducing a character or setting, but rather by introducing a knife. Right from the beginning, you know this is a story in which threats are real, and the anticipation of what's going to happen keeps you reading. As the chapter continues, readers are introduced to Jack, the man who wields the knife, who has killed the entire family living in a home he's invaded, except for the youngest child. We enter the story as he's moving to murder the baby, but, as we find out, the baby is not in the crib.
The hook, surprisingly, is not the murder; that takes place before the story begins. Instead, the hook is the mystery of where the baby has gone, why it's so important to Jack to find him, and why the murder took place at all. The opening scene sets up many questions, so readers want to keep reading to find out the answers. The moment when Jack discovers the baby is missing serves as the hook for The Graveyard Book, as it takes a scene with a finite ending where all the characters except Jack are dead and makes it a mystery instead, driving readers to continue.