From Introduction:
Traveller is a hard-SF (Science-Fiction) roleplaying game. That is to say, the Traveller universe obeys consistent and realistic laws. A few physics-tweaks are necessary to allow characters to travel between worlds, but what they find on those alien worlds will be physically plausible.
Traveller also obeys the laws of social science. The people encountered in the game will behave in a believable manner. Even aliens will follow their own consistent rules of behavior, even if they are so alien as to be incomprehensible.
Far from limiting the imagination, this grounding in hard science promotes creativity by creating a sound foundation to work from and a core of “normality” to counterpoint the wonders of the universe. Nothing is weird for the sake of making it weird, but there is certainly plenty of strangeness to explore and encounter.
The Traveller universe is a strange and mysterious place, but it is not frivolously so. A mysterious artifact or an alien society will have a reason for being as it is. Finding that reason may be a challenge, but who said exploring the universe was going to be easy?