The Imperium survived for more than a thousand years. In less than a minute, it has shattered. An assassin has cut down the emperor and his heirs in one bold strike, throwing the largest interstellar empire in history into chaos. There are many candidates for the empty throne, and the worlds of the Imperium are divided in their allegiances. By a curious twist of fate, the assassin himself now controls three rebellious sectors. The emperor's nephew, considered by many to be a murderer himself, has assumed the throne over the objections of the Imperial Assembly. Star fleet battles rage throughout the disputed territories. As sectors are stripped of their fleets to fight in the rebellion, imperial territory is invaded by alien forces sensing weakness and plunder. Secter after sector secedes from the Imperium to manage its own defense. Sector after sector falls to external forces.
And on the start of rebellion and interstellar chaos, Traveller sets an ongoing drama of role-playing adventure... Players travel from star system to star system, taking sides in the rebellion, fighting for their cause while fighting to stay alive. The dangers of the interstellar environment combine with the dangers of a shattered Imperium to challenge players and referees and provide never-ending excitement in the far future.
The Imperial Encyclopedia. This book is a reference for both players and referees. It includes essays on the Imperium and other interstellar empires, descriptions of starships and vehicles in common use throughout known space, lists of equipment available on the open market and its prices, maps and other information valuable to players and referees alike.
There's a wealth of information about MegaTraveller in the DTRPG Guide to MegaTraveller (free download).