Stories about space, technology, the future, and other things that could be real but aren't.
The exact definition of science fiction is controversial. Generally, if what's happening in the story is impossible, but explained by scientific principles, it will qualify as sci-fi. "Hard" sci-fi focuses on strict extrapolation from contemporary scientific theories. "Soft" sci-fi is more apt to use hand-waving science in order to move the plot forward. Social sci-fi focuses not on technological advancements, but on the social, political, and ethical implications thereof. If the impossible things in your story are only explainable by magic, toddle over to the #fantasy tag.