Fantasy is a genre of fiction (make believe) that shows some form of magic or supernatural elements.
Often fantasy also means that the story happens in a fictional place, a world different from our own (e.g. Middle-earth or Narnia). Although the word fantasy is most often applied to low-tech worlds, it also can depict high tech worlds, overlapping with science fiction (science fantasy). Or it is used for historical fiction, if it is magical in some way.