Suppose the accident on the previous slide is analysed and every factor relating to it is represented by a domino standing on end. There may be many dominos but I've only shown four. Let's imagine the forklift was being driven too fast and that is represented by the domino on the right.
The truck went over a pothole so that's number two. The load was loose so that becomes number three and the end result is represented by domino number four.