You’re whizzing along the highway, when the driver of the car in front of you suddenly slams on the brakes, perhaps to let another vehicle merge into the lane. You respond by braking, too, as does the driver of the car behind you, and so on. And your tendency is to brake harder—just to be on the safe side—than the driver in front of you did.
This effect can propagate backward for long distances, depending on the traffic volume and speed, creating waves of congestion.