贵姓 (guì xìng) is a polite, fixed phrase meaning 'your honorable surname?' — it's the respectful way to ask someone's surname in Chinese, especially on first meeting. Unlike 姓 (xìng), which is the neutral word for 'surname' and can be used in statements or questions, 贵姓 is only used as a polite inquiry and includes the honorific 贵 (guì 'honorable'). 姓氏 (xìng shì) is a more formal, written term for 'family name' and is not used in direct questions.