1. What does it mean?
극락 has a Buddhist religious meaning, but casual reactions also use it for an exceptionally comfortable or delightful experience. A soothing voice, delicious bite, or perfectly timed rest can be described as 극락.
Also written or said as geungnak
2. The nuance
This figurative use expresses personal pleasure, not a theological claim. It can be enthusiastic fan language, but is not restricted to an idol's appearance.
3. Examples
“추운 날 따뜻한 방에서 이불 덮으니 극락이다.”
Being under a blanket in a warm room on a cold day is pure bliss.
“두 목소리가 겹치는 마지막 화음은 극락이네.”
That final harmony with the two voices overlapping is heavenly.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.