Ask any successful person to look back over the events of his or her life, and chances are there'll be a turning point of one kind or another. It doesn't matter if that success has come on a ball field or in a boardroom, in a research laboratory or on a campaign trail - it can usually be traced to some pivotal moment.

This quote is saying that if you ask any successful person about their life, they will usually be able to point to one significant moment when their life changed significantly. This moment would have been an important turning point, and led them to their current success, regardless of the field they are in (sports, business, science, politics, etc). This point of time triggered an important change of direction and enabled them to become successful.