In this use, SF is a short written form of science fiction: stories about imagined science, space, other worlds, future machines, and what people might do with them. Bookshops, fan clubs, and review pages use SF a lot; it is a label on a shelf, not a word you must say out loud. When people speak, they more often say science fiction or sci-fi. Sci-fi can sound a bit pop and loose; SF can sound a bit more “this is the genre, seriously,” though plenty of readers use both for the same books. SF is not a spoken sentence on its own — you still need a noun: an SF novel, SF films. Other meanings of the letters SF exist in other worlds (a city, a file type), but here the filing is the story genre.