Slughorn mentions that Amortentia has a distinctive "crack" when its stopper is removed, and this sound becomes significant as the book progresses. Harry later discovers a book filled with notes and instructions written by the "Half-Blood Prince," and he notices that the book is filled with references to this "crack."
The crack turns out to be a key ingredient for a powerful potion, and Harry learns that Voldemort is desperately searching for it as well. It is eventually revealed that the crack is actually the essence of a unicorn, and Voldemort wants it to strengthen his soul and make himself immortal.
The crack plays an important role in the story's climax, and its significance adds to the overall intrigue and suspense of the book.