Traffic Jam Assist uses a camera mounted on the windshield, plus a radar embedded in the front, to watch what's ahead on the road. In a heavy traffic situation, the system can take over the constant stopping and starting that can fatigue or annoy a human driver. At a stoplight, it can even move the car forward based on watching the car ahead of it--but it can't do that if it's the car at the front of the queue.
Autonomous driving features like this are slowly making their way into cars, as they have been for many years. This system seems shocking now, but by the time it's fully developed, we could already be used to the idea of computers' taking such an active role in driving the car.
For now, Traffic Jam Assist is still working on a few kinks that are also challenge for other such systems. It can't turn corners on its own. Also, if the lane markings are faded or otherwise obscured, the car may not be able to keep its lane. When all conditions were met, however, it was surprisingly good at maintaining a steady speed and distance from other cars--more consistently than a human could.