1. What does it mean?
대박 can describe a major success or act as an exclamation when something is impressive or surprising. In casual speech it can also intensify a description, much like 'really' or 'super.'
Also written or said as daebak
2. The nuance
A cheerful 대박 usually signals admiration, but a stunned tone can signal disbelief at bad news too. Translate the reaction in context instead of treating every 대박 as a positive review. It is conversational, not a formal assessment.
3. Examples
“네 그림이 전시회에 걸린다고? 대박, 축하해!”
Your painting is going to be exhibited? Wow, congratulations!
“이 작은 가게에 사람이 이렇게 많다니 대박이다.”
I can't believe this little shop has so many people in it.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.