From this book review:
Taleb is concerned with black swans, i.e., unpredictable and improbable events that have great impact. Among the examples of these he cites are the rapid spread of the Internet and the 9/11 attacks.
People endeavor to explain black swans after they occur, but they cannot do so in advance. Despite the crucial effects of these events, economists and other supposed experts in prediction fail to allow for them; indeed, their theories often deny their possibility.
Because of this failure, Taleb maintains that much business forecasting is useless. To him, only a few economists, e.g., Friedrich Hayek, grasp the vital importance of uncertainty and escape condemnation. Taleb extends his indictment of conventional approaches to risk, contending that the bell curve, a key tool of many standard theories, often fails to fit the actual world. He further posits that people tend not to cope with a black swan properly, tailoring their response to specific details of the incident rather than generalizing their response.