Traveller is a role-playing game. MegaTraveller is the name of this Traveller rules system set against the background of star-spanning rebellion in the Imperium.
The Referee's Manual. This book is intended as a rules reference for the Traveller referee. It includes coverage of vehicle and starship design, starship combat, world and star system generation and description, animal encounters, trade and commerce, detailed task resolution, and administration of adventures.
Other Traveller Rules Books. This book is part of a three-book set which provides basic rules and background for playing the game.
The compaion Players' Manual is a basic compilation of Traveller rules and concepts for the individual player. It includes coverage of character generation, combat resolution, psionics, and tasks.
The companion Imperial Encyclopedia is a basic reference for both players and referees. It includes historical essays on the Imperium and other interstellar empires, descriptions of starships and vehicles, lists of equipment and prices, maps of territories, and other information valuable to players and referees.
There's a wealth of information about MegaTraveller in the DTRPG Guide to MegaTraveller (free download).