This book does an excellent job of bringing the classic Traveller rules into the 21st century. It is an intensive rewrite of the old Traveller Starter Rules from the early 1980s, and it introduces many "modern" gaming elements that make the game more fun, and easier to play, while maintaining the flavor of Traveller in the 3rd Imperium setting.
Like the original rules, they can be used for developing a new setting, or for modeling a different setting. In fact, Mongoose did this themselves, bringing out several good settings for this version of Traveller including Babylon 5, 2000AD, and Hammer's Slammers.
While the supplemental materials published for this edition of Traveller later vary greatly in quality, this core rule book contains an entire game on its own and can be used as the sole book for play. Careful selection of adventures and additional books will allow it to be supplemented. But really, this book is not only all you need, it has enough to form the basis of an excellent long-running campaign.