번개 is a slang for 즉석 모임 which literally means 'impulsive meeting' which would mean spontaneously making plans to meet with someone. 번개 is also used in internet Clubs. For example, the members chat on internet and someone says
A : "Why don't we meet in person? Let's go grab a beer."
B : "That's great!"
C : I'm in.
D : I'm in.
A : "OK. Hey guys! Let's meet at Gangnam station right now!"