This is the 4603rd most frequent Japanese word.
息子
Son.
The word "息子" refers to someone's male child here, specifically identified as "his son".
彼の息子はとても優秀な学生です。
His son is a very excellent student.
In this sentence, "息子" refers to the speaker's own male child.
息子が初めて歩いた日を覚えています。
I remember the day my son took his first steps.
Here, "息子" is used to describe the friend's male child.