This is the 709th most frequent Korean word.
만난
"Met" or "encountered."
Here, '만난' is the past participle form of '만나다' (to meet), used to describe '날' (day) as the day on which the event occurred.
우리가 처음 만난 날이 기억나요.
Do you remember the day we first met?
In this sentence, '만난' modifies '친구들' (friends), indicating the friends are the ones met earlier or previously encountered.
만난 친구들이 도와줬어요.
The friends I met helped me.
'만난' describes '일' (fact/matter) to indicate the specific event among others that is significant.