In this passage, Suvin attempts to further differentiate science fiction from all other genres. "Necessary and sufficient conditions" means that science fiction has rules that make it the genre that it is. These rules are specific and exclusive to science fiction, making science fiction its own special genre. Several of these rules are as follows.
First, science fiction is different from realism because it incorporates estrangement, the literary device of timelessness.
Second, science fiction is different from myth because it handles timelessness in a different way: with changes that coincide with the story's setting.
Third, it is different from the folk (or fairy) tale because science fiction uses the literary device of imagination differently. Whereas the world of the folk tale is impossible, the science fiction world is most likely a possible, imagined future.
And finally, it is different from fantasy because science fiction is based on science and possibility, whereas fantasy is rooted in the supernatural. Fantasy goes even further into the realm of impossibility than the folk tale does. Some things in a folk tale don't adhere to how our reality works, but fantasy turns reality upside down completely. Fantasy rejects reality. The folk tale doesn't care about it. Science fiction, though, works with reality in a new, imagined way.
Fantasy, like these aforementioned genres, deals with the past or present, whereas science fiction deals with the present and future. A work of fantasy is a world of the impossible, whereas science fiction is the world of the improbable. This is a huge difference. Suvin is against the conflation of all these categories, which he believes are their own separate entities.