SF
SF has 2 meanings
Science Fiction
When you're discussing fictional stories, SF stands for "science fiction" (or more colloquially, "sci-fi"). SF stories feature some combination of futuristic technology, aliens, space travel, time travel, parallel universes, and other advanced or speculative scientific concepts.
Many of the world's most popular media franchises tell SF stories. For example, you might have heard of Star Wars (SW) - the world's most popular SF franchise. Star Trek is another popular SF property (just ask your favorite Trekkie), and the MCU includes many SF stories.
Of course, not all SF is part of a multimedia juggernaut. You can find additional, standalone SF tales in your local bookstore's science fiction section, as well as on many streaming services.
Example
My favorite SF stories all have to do with time travel
Take the pop culture acronym challenge
A baseball acronym that refers to the number of sacrifice flies hit by a player.
A sacrifice fly is recorded when a hitter hits a ball that is caught for an out but advances a runner on the base paths. It is considered a "sacrifice" because the hitter gets out in order to move his teammate closer to scoring.