It's a formal way to ask someone who they are, and it's commonly used in situations where you don't know the person. You can also use it when you've forgotten someone's name.
Here are some additional points:
* It's considered polite to add "세요" (se-yo) to the end of the question when speaking formally.
* You can use a less formal version, "누구야?" (nu-gu-ya) with close friends or family.
* It's often used in conjunction with other phrases, like "저는..." (I am...) or "제 이름은..." (My name is...).
Here are some examples of how to use "누구세요" in a sentence:
* 누구세요? (Who are you?)
* 누구세요? 저는 김민수입니다. (Who are you? I am Kim Min-su.)
* 누구세요? 이름이 뭐예요? (Who are you? What's your name?)