Green light is a slang often used by criminals to give the go ahead to commit a crime, more specifically carry out a hit on someone. Though the meaning of green light differs between certain criminal organizations, they're all giving the go ahead to proceed with an action.
In the Mafia, people whom have been made a 'made man' can't be killed without authorization from high ranking members, thus if anyone wants to whack a made man in the mafia, it's necessary for them to receive a green light before proceeding with killing them.
Green light is a term also used to put a hit on someone, if a gang member say someone has been green lighted, it means that the individual has a hit out for them. In 2014, Game, a rapper and Bloods gang member who's been feuding with rival Crip rapper '40 Glocc', posted a green light traffic light photo on his instagram page with the captions 'Green Light'. Many believe that was the Game's way of putting a hit out on 40 Glocc.
The term 'green light' is not used exclusively among criminals, it's often used by non-criminals to give the go-ahead. For instance, a project manager might give the green light to proceed with a project.