"One of the questions Snape asks is if Harry knows what would result in adding the powdered root of asphodel to an infusion of wormwood. Harry doesn't know the answer, but that's not the important part.

This is actually a coded comment that would be lost on all viewers that don't have an extensive knowledge of Victorian-era flower symbolism. Asphodel is a lily, which means demise under the Victorian flower code, while wormwood reflects loss and absence. Snape is actually referencing Harry's mother Lily with this comment"