I am not exactly sure whether you want to know about the default setting, or about the sort of game Traveller is. I'll take a swing at the latter.
Traveller is about that sub-genre of SF called interstellar SF. Interstellar SF stories concentrate on civilizations, usually human, but occasionally alien, who use Faster-Than-Light technology to travel the stars. Most early SF (the Golden Age of the late thirties and forties, and extending into the fifties and sixties, and even later) is actually interstellar SF.
So the sources for Traveller include the writings of Isaac Asimov, H. Beam Piper, Poul Anderson, Jack Vance, Alexi Panshin, Keith Laumer, Robert Heinlein and others.
Space Opera is sometimes considered to be interstellar SF, but is thought of as a poor cousin, with a distinct pulp sensibility. Traveller leans away from this, and is usually considered to be 'Hard' SF, or more scientifically accurate SF. That's probably true as far as RPG SF goes, which is 'pulpier' than regular SF. There are definite pulp aspects to Traveller, but the assumed background looks more like Serenity than Star Wars.
Characters in Traveller travel the stars. That’s the raison d’être for the game, and is the basis of all other activities in Traveller. The rules specifically address a host of issues dealing with FTL travel, including trade, naval combat, the problems of creating worlds for players to visit, developing tools for creating alien life forms for the players to interact with. For example, many of the skills don’t make sense outside the context of a starship.
The original rules do assume as common technology, society and technological development. These elements can be changed while still using the rules, but the amount of work required depends on how far you stray from the baseline technology.
For example, if you decide to use a different FTL paradigm than the one offered by the game (Jump Drive), you may have to change some of the assumptions about the way a society would develop. Much as D&D offered a template for a particular style of Swords and Sorcery, Traveller offered a template for a distinct style of SF, which most referees used, even if they did not rely on the specific Traveller Universe background developed for the game.
Any other questions?