1. What does it mean?
절친 describes a close, trusted friend. It is related to 절친하다, meaning to be very close, and often appears when people talk about enduring friendships or familiar pairs.
Also written or said as 절친한 친구
2. The nuance
Compared with 친구, it emphasizes closeness; compared with 베프, it need not suggest a single 'best' friend. It is fine in everyday descriptions, but the label alone cannot tell you how often two people meet or share private matters.
3. Examples
“중학교 때 절친을 십 년 만에 다시 만났어.”
I met a close friend from middle school again after ten years.
“우리는 취향은 달라도 서로 잘 이해하는 절친이야.”
We have different tastes, but we're close friends who understand each other.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.