SF is often about the future. It can be about imaginary new science and inventions such as spaceships, time machines, and robots. Science fiction stories are often in a world that is very different from the real world. They can have science and tools that do not exist in reality. Science fiction stories often take place on other worlds. There are often alien creatures.
Science fiction is drastically different from fantasy. Fantasy stories often have magic and other things that do not exist and are not science. Isaac Asimov was a famous science fiction writer. He once said that science fiction is possible, but fantasy is not.
Writers often use SF to explain everyday questions or problems by putting them in the future. Usually they invent a very different world to help people notice important ideas.
Science fiction changes over time. Some authors wrote SF books before this type of writing had a name. These writers and books were not called science fiction when they were published. But, they are often called science fiction today.