Green wave is an intentionally induced phenomenon in which a series of traffic lights (usually three or more) are coordinated to allow continuous traffic flow over several intersections in one main direction.
Any vehicle travelling along with the green wave (at an approximate speed decided upon by the traffic engineers) will see a progressive cascade of green lights, and not have to stop at intersections. This allows higher traffic loads, and reduces noise and energy use (because less acceleration and braking is needed). In practical use, only a group of cars (the size is defined by the signal times) can use the green wave before the time band is interrupted to give way to other traffic flows.
The coordination of the signals is sometimes done dynamically, according to sensor data of currently existing traffic flows - otherwise it is done statically, by the use of timers. Under certain circumstances, green waves can be interwoven with each other, but this increases their complexity and reduces usability, so only the roads and directions with the heaviest loads get this preferential treatment.
Additional: what are the physics of a green wave breakdown?