Plain English Meaning
A weather alert that warns pilots about thunderstorms or thunderstorm activity bad enough to be dangerous to any aircraft. It tells you where the worst storms are, how strong they are, and how long the warning is good for.
Full Definition
A weather advisory issued by the Aviation Weather Center for hazardous convective weather that is significant to the safety of all aircraft. A Convective SIGMET is issued for severe thunderstorms with surface winds of 50 knots or greater, hail at the surface 3/4 inch in diameter or greater, tornadoes, embedded thunderstorms, , or thunderstorm covering at least 40 percent of an of 3,000 miles or greater with thunderstorms reaching above a specified . are issued hourly for the eastern, central, and western and are valid for two hours or until superseded.