An amendment is a change made to an original, usually a document or statement. You might propose an amendment to your environmental club's bylaws requiring that all official club documents be printed on recycled paper.
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.