An amendment is a change made to an original (usually a document or statement). It's meant to improve the original, but one could say the 21st Amendment (prohibition) did more to make Al Capone rich than to make Americans teetotalers.
Can you spot the word amend, meaning "to make better" or "to cure of faults and errors" hidden in amendment? If so, you'll grasp the notion that an amendment is something intended to improve whatever preceded it. It pops up most often in formal legal contexts, where it refers to a written change to a bill, law, contract, or the U.S. Constitution.