What's Science fiction all about?
It's a genre, sometimes shortened to sci-fi. Usually all about futuristic science and technology. Stuff like time travel or space travel for example.
Why the use of bother terms science and fiction,why not simply science or fiction?
Well because it combines both science and fictionis called literature of ideas. It combines consequences of scientific experiments. 
So are they related to movies?
Sure, Science Fiction is both a movie and literature genre. HIstorically it has grounding in actual science but nowadays more of a fiction.
What what sci-fi movie do you know of ,may be that could explain the concept better
Science fiction film
Science fiction film (or sci-fi film) is a genre that uses speculative, fictional science-based depictions of phenomena that are not fully accepted by mainstream science, such as extraterrestrial lifeforms, alien worlds, extrasensory perception and time travel, along with futuristic elements such as spacecraft, robots, cyborgs, interstellar travel or other technologies. Science fiction films have often been used to focus on political or social issues, and to explore philosophical issues like the human condition. In many cases, tropes derived
Science fiction
Science fiction (sometimes called Sci-Fi or simply SF) is a genre of speculative fiction that has been called the "literature of ideas". It typically deals with imaginative and futuristic concepts such as advanced science and technology, time travel, parallel universes, fictional worlds, space exploration, and extraterrestrial life. It often explores the potential consequences of scientific innovations.
Science fiction, whose roots go back to ancient times, is related to fantasy, horror, and superhero fiction, and includes many subgenres