The way science fiction works is that you start out with the existing rules
of the world that we're in, and you add some technology that remains consistent
with it.

Now, there's a lot of technology that SF readers are generally familiar with.
If you read a story from the thirties about faster than light travel or
instantaneous communication, they'll explain it.  But today, the reader is
expected to understand a lot of that background, so the author can just wave
his hands and say "FTL" or "it's an ansible" and expect the audience to catch
on.  This is part of the problem with people just starting to read SF; they
don't have some of this background and aren't sure how to get it.

But when you DO introduce a new technology, you explain it, and you DO NOT
surprise the reader by changing all the rules on the fly (like suddenly
introducing something that changes all the rules we thought we knew).

This is distinguished from a fantasy story, in which the rules of the world
we live in do not exist, and are replaced with a different set of rules.  
In this case, you can get away with nearly anything as long as it is
consistent and you can suddenly change everything and explain "the magic
helmet did it" and get away with it.  You cannot do that in SF.

The problem with a lot of science fiction television and movies is that
they are written and produced by people who have no clue about the rules
mentioned above.  For a good discussion of how this sort of thing comes
about, read Harlan Ellison's description of the TV show _The Starlost_
in his book The Second Glass Teat.  It's hilarious.
--scott