Fantasy set in the real world (or a version of it) where magical elements intrude into everyday life.
Low fantasy grounds its story in the real world or a setting closely resembling it, then introduces magical or supernatural elements that disrupt the ordinary. The magic is often rare, dangerous, or hidden. The stakes tend to be personal rather than world-ending, and the contrast between the mundane and the magical is a major source of tension.
Low fantasy lets you write about magic without building an entire world from scratch. It's also a great vehicle for metaphor, since magic intruding on reality can stand in for anything that disrupts ordinary life. Understanding low fantasy helps you decide whether your story needs a secondary world or whether reality with a twist serves the concept better.
A hidden magical world beneath London, accessible only to those who 'fall through the cracks' of ordinary society.
Ancient, terrifying magic invading a quiet English childhood, with reality and myth bleeding into each other.
A mysterious labyrinthine house that may or may not exist alongside our reality, blurring the boundary between low and high fantasy.
The 'low' refers to the amount of magic in the setting, not the quality of the writing. Low fantasy is a classification, not a judgment.
Part of low fantasy's power is the contrast between ordinary and extraordinary. If magic is everywhere, you've drifted into a different subgenre.
Write a scene where something impossible happens in the most ordinary setting you can think of: a grocery store, a dentist's waiting room, a bus stop. Keep everything around the impossible thing completely normal. Let the contrast do the heavy lifting.