This is the 11346th most frequent Korean word.
김윤서는
"김윤서는" is not a standalone word but likely refers to a Korean name: Kim Yoon-seo.
Here, "김윤서는" functions as the subject of the sentence, indicating the person who is studying.
김윤서는 공부하고 있습니다.
Kim Yun-seo is studying.
Here, "김윤서는" is used as the subject of the descriptive sentence, describing Kim Yun-seo as being a good person.
김윤서는 참 좋은 사람이에요.
Kim Yun-seo is a really good person.
In this sentence, "김윤서는" is again the subject, referring to the person performing the action of lending a book.