1. What does it mean?
그린라이트 borrows the traffic-light image of a green light. In dating conversation it means a signal interpreted as encouraging further romantic interest.
Also written or said as geurillaiteu
2. The nuance
It is an interpretation, not proof or consent. An invitation or friendly message can have nonromantic reasons; ask and respect the person's answer instead of treating a checklist as certainty.
3. Examples
“다음 전시도 같이 보자는데 그린라이트일까?”
They suggested seeing another exhibition together. Could that be a sign of interest?
“친절한 답장을 그린라이트로 단정하지 않으려고 해.”
I'm trying not to assume a friendly reply means romantic interest.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.