This is the 9174th most frequent Japanese word.
父さん
"Father" (informal, affectionate).
Here, '父さん' is used to refer to one's own father, indicating a family relationship.
父さんは昨日、ゴルフに行きました。
My dad went golfing yesterday.
In this sentence, '父さん' is used affectionately to specify the speaker's father and the occasion of his birthday.
今日は父さんの誕生日だから、ケーキを作りました。
Today is my dad's birthday, so I made a cake.
Here, '父さん' is casually referencing the speaker's father when talking about his activity of watching a movie.