The phrase "Amy has an aptitude for math. Her friends envy her natural ability" suggests that Amy possesses a natural talent or skill in mathematics, which is something that her peers admire and perhaps wish they had themselves.
CLUE: "Aptitude" indicates a natural ability or talent in a particular area (in this case, math).
MEANING: The friends' envy reflects their recognition of her skill and possibly their own struggles with mathematics, highlighting the contrast between her natural ability and their experiences.