1. What does it mean?
인생템 combines life with item. It describes a product that fits someone's needs so well that they regard it as an exceptional long-term favorite, rather than merely the latest trend.
Also written or said as insaengtem
2. The nuance
It is a strong personal endorsement, not a lifetime guarantee or proof that it will suit everyone. Sponsored use of the phrase is still promotional language.
3. Examples
“몇 년째 같은 펜을 쓰니 내 인생템인 것 같아.”
I've used the same pen for years, so I think it's my holy-grail item.
“남의 인생템보다 내 손에 편한 걸 골랐어.”
I chose what felt comfortable in my hand rather than someone else's favorite.
Sources
Sinjoeo Korean Slang Dictionary
References support the meaning and usage. Example sentences are written for NoCapWiki.