SF has its own terminology. Back in the late 60's and early 70's sad geeks went around speaking a whole different vocabulary - but this has died out leaving us with a few good words and phrases.
If it is done well it is so seamless that the reader hardly notices, done badly and it breaks the flow of the story. This is dubbed "InfoDump" - the most obvious of which is the old "So tell me, professor, just how does the time machine work?" ploy.
Some SF authors are better than others at this. Frank Herbert's "Dune" is a good example of the use of narrative whereas the late great Isaac Asimov seems to have taken great delight in giving his characters long explanations to spout.
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