You're successful in your business. You're held in high regard by others as intelligent, capable, and a force with whom to be reckoned. Yet in the middle of the night or during the day, the thought comes to you: What if they realize I'm a fake? What if they realize I'm not that (you fill in the blank... smart, talented, capable, witty, whatever).
You are one of the many people who are struck with the impostor syndrome.