This is the 4173rd most frequent Japanese word.
一男
"一男" is a Japanese male given name meaning "first son."
In this sentence, '一男' is used as a proper noun, specifically as a male given name.
一男は学校へ行きました。
Kazuo went to school.
Here, '一男' refers to the subject of the sentence, denoting a specific person's possession.
一男が持っている本は新しいです。
The book Kazuo has is new.
This usage of '一男' indicates someone's given name, introduced in a sentence.