Phrases - 开绿灯(kāi lǜ dēnɡ)
开绿灯, literally means "turn on the green light", is now used to indicate "giving permission".
As you know, traffic light are usually composed of a set of red, yellow and green lights. Vehicles should stop at the red light and go at the green light. If you "turn on the green light", that gives the driver a signal to go. Later, this term was expanded to refer to the situation when someone is given permission to do something. Sometimes the term is used in a derogatory way, when a green light is given to someone for something that is not entirely legal or appropriate.