This is the 11326th most frequent Korean word.
김수일이
The term "김수일이" does not appear to have a specific definition in standard Korean. It might be a proper noun, such as a person's name (Kim Soo-il followed by a subject particle).
In this sentence, '김수일이' is the subject of the action described by the verb '갑니다', using the subject marker '이'.
김수일이 학교에 갑니다.
Kim Soo-il goes to school.
Here, '김수일이' indicates the direct object of affection, where '이' is in conjunction with the verb '좋아합니다'.
나는 김수일이 좋아합니다.
I like Kim Soo-il.
In this context, '김수일이' represents the doer of the action in the subordinate clause, taking the subject marker '이'.