The term "a perfect storm" is used to describe a serendipitous confluence of events which results in something astounding and often catastrophic. Considered on their own, each of the events is not terribly remarkable, but when the events are combined, the results can be formidable. The term is used both literally, to refer to ongoing events, and hypothetically, to talk about potential disaster scenarios.
Use of this term is not restricted to the field of meteorology, although the term has its origins in meteorological phenomena. Many people in the field of politics, for example, refer to a perfect storm of events which causes an unexpected or shocking result, as do economists. The catastrophic flailings of the American markets in 2008, for example, were sometimes said to be caused by a perfect storm of events which united to create chaos.
Credit for the term is usually given to Sebastian Junger, who wrote a book called The Perfect Storm in 1991. The book was about a catastrophic 1991 storm which caused significant amounts of damage across the Eastern Seaboard of the United States, and it was even made into a film. Junger, however, points out that he borrowed the term from officials at the National Weather Service, who referred to the events which led to the 1991 Halloween Nor'Easter as "perfect" to create a devastating storm.
In disaster scenarios, officials try to consider the conditions which could lead to a perfect storm, so that they can anticipate such events and theoretically cope with them. The infamous 2005 hurricane season in the Atlantic is often used as an illustration of a perfect storm, as much of the Southern United States was battered by a series of devastating hurricanes, in a series of events which no one had predicted. While just one hurricane's damage was manageable, the combined effects of Emily, Dennis, Rita, Katrina, and Wilma proved to be too much for many Southern States.
People may also use the term when looking back on historical events. It is often easier to construct a perfect storm after the fact than to predict one, as historians can take advantage of the fact that they have the big picture to look at. A perfect storm of events can lead to everything from the collapse of a government to the failure of an economic market, sometimes alarmingly quickly.