清 = 'clear / clean', 秀 = 'elegant / graceful'. Describes an understated, delicate beauty - usually a face (清秀的脸) or handwriting/artwork (字迹清秀). Contrast with 漂亮 (pretty, general) or 艳丽 (gaudy, showy).
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Tends to be used about women, children, and young men with refined features. The classic gloss is 清爽秀丽，不俗气 - 'clean, elegant, not vulgar'. Considered a classy, literary compliment.
