Science fiction is a broad, very broad genre. This implies that it’s difficult to define what science fiction, and sometimes genres such as fantasy are miscategorized as science fiction. However, it’s commonly accepted that SF stories are set in either the near or more distant future. But not all stories set in the future are science fiction: if you write a story about the 2016 US Presidential Elections, it’s clearly set in the future but if you assume that science and technology will not change much in the next three years; we can’t hardly consider your story as science fiction. A key aspect of science fiction is the evolution of science and technology, often set in the future.
As a general definition, but not a perfect one, we might say: Science fiction is the literary genre which revolves about scientific and technological progress, often set in the near or distant future. Then what is mundane science fiction? According to Wikipedia is mundane science fiction:
a sub-genre of science fiction, similar to hard science fiction, which is characterized by its setting on Earth or within the solar system, and a lack of interstellar travel or contact with aliens. […] It focuses on stories set on or near the Earth, with a believable use of technology and science as it exists at the time the story is written.
Most of the stories I will place, follow this description. However, sometimes I will deviate from this description, but only to a limited degree. My favourite topic, space colonization, is a believable extension of current technology, albeit it currently not being fully developed. Therefore it fits into the definition of mundane science fiction.