1. What does it mean?
스포 shortens 스포일러, borrowed from 'spoiler.' It refers to revealing a plot point, result, or surprise before someone has experienced it.
Also written or said as 스포일러, 스포 주의, spoiler
2. The nuance
It applies beyond films: a game ending or competition result can also be a 스포. What counts as too much information depends on the audience, so a 스포 주의 warning is useful before details. A vague reaction is not always a spoiler.
3. Examples
“아직 마지막 회를 안 봤으니까 스포는 하지 마.”
I haven't watched the final episode yet, so don't spoil it.
“후기 앞에 스포 주의라고 적어 두었어.”
I put a spoiler warning at the start of my review.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.