姝 is a classical literary term meaning 'beautiful' or 'a beautiful woman'. It is a bound form (b.f.) in modern Chinese, meaning it almost never stands alone as a word — you'll find it in classical poetry, set phrases, and women's names (e.g., 林姝). It is not used in everyday conversation; for 'beautiful' in modern speech, use 漂亮 or 美丽.