Oyasumi nasai means "Good night." This phrase is used when you are saying goodbye to someone for the night, especially before going to bed. It's a polite and gentle farewell. You can say this to family members or close friends. For example, "もう寝るね。おやすみなさい！" (I'm going to bed now. Good night!).

夜(よる)、寝(ね)る前(まえ)に別(わか)れる時(とき)に使(つか)う挨拶(あいさつ)です。家族(かぞく)や親(した)しい人(ひと)に言(い)います。「おやすみなさい。」