An idiom meaning to teach someone something they already know well (teaching your grandmother to suck eggs; preaching to the choir).
彼にそんな基本的なルールを説明するのは、釈迦に説法だよ。
Explaining such basic rules to him is preaching to the choir — he already knows them.
料理長の前で包丁の持ち方を注意するなんて、釈迦に説法だ。
Pointing out how to hold a knife in front of the head chef is teaching the teacher.
新入社員がベテランに対して細かく指導するのは釈迦に説法になりかねない。
A new employee giving detailed instructions to a veteran can come off as preaching to the choir.