In hard science fiction, science is very prominent in the story.  It is almost as if it is a "main character."  The story is apt to be dependent on -

     some scientific discovery

     some scientific device

     science and the scientific method saving civilization as we know it

Hard science fiction stories often ask the question "What if..." and then go on to describe the consequences of the science in the story.

A good place to find hard science fiction is the magazine now known as "Analog Science Fiction & Fact" (it used to be known as "Astounding Science Fiction."