This is the 11714th most frequent Korean word.
났던
"났던" means "that had occurred" or "that happened" (past tense).
The word '났던' is used here to indicate a past occurrence (the rainbow appearing) being described in the sentence.
어제는 하늘에 무지개가 났던 날이에요.
Yesterday was the day when a rainbow appeared in the sky.
The word '났던' illustrates a momentary past event (having mentioned the goal).
그는 어릴 때 꿈이 있었는데 지금 기억나는 건 내가 말했던 목표였다.
When he was young, he had a dream, but now what I remember is the goal I had mentioned.
'났던' conveys the act of having received the award as a past completed action.