Science fiction is a pretty broad genre, and seems to get wider every year. Novels are categorized as dystopian, first contact, military, space opera, steampunk, horror, space western - and on and on. But another way to subdivide science fiction uses a scale that ranges from hard (a story closely tied to reality, as we currently understand it) to soft (a story with plenty of hand waving and few explanations).
My favorite scale of hardness uses time travel as an example:
In soft SF: "You sit in this seat, set the date you want, and pull that lever."
In medium SF: "You sit in this seat, set the date you want, and drive to 88 mph."
In hard SF: "A good question with an interesting answer. Please have a seat while I bring you up to speed on the latest ideas in quantum theory, after which I will spend a chapter detailing an elaborate, yet plausible-sounding connection between quantum states, the unified field theory, and the means by which the brain stores memory, all tied into theories from both Albert Einstein and Stephen Hawking."
In really hard SF: "It doesn't. Time travel to the past is impossible."
Of course, there are gradations. For example, I start my books as firmly in the hard category as I can, but loosen a bit toward medium when the story needs more traction. You might call this a medium-hard approach.
The science fiction hardness scale tells you something about the style of the book, but bears no relation to its success in entertaining readers. A hard science fiction story is not "better" than soft, it could be so bound by real-world limitations that it becomes deathly boring. Likewise, soft is not "bad". Star Wars is certainly soft science fiction, with X-Wing fighters screaming and banking through the vacuum of space as though flying through air. But it's hard to argue with the movie's ability to entertain, so we set aside our overly-analytic brains and simply enjoy the ride.