Weird fiction is the genre for stories that refuse to be anything else. It predates modern genre boundaries: not quite horror, not quite fantasy, not quite science fiction, but drawing from all three. The defining quality is strangeness that resists explanation. Something is deeply, fundamentally wrong, and the narrative doesn't reassure you by explaining what. The tradition runs from M.R. James through Lovecraft to modern writers pushing the form's boundaries.
Weird fiction teaches you to embrace ambiguity as a tool. In a publishing landscape that demands clear genre labels, understanding weird fiction gives you permission to write the uncategorizable. It also trains a specific skill: making the reader uneasy through what you don't explain rather than what you do.
Two travelers on the Danube encountering something vast and inhuman in the landscape, one of Lovecraft's favorite stories.
An expedition into an inexplicable zone where biology has gone wrong in beautiful and terrifying ways.
Profoundly strange fiction about adopted children raised by a god-like figure, defying all genre classification.
Write a scene where something is wrong, but you never identify what. A character walks through a familiar place and notices details that are subtly, inexplicably different. Don't resolve it. Don't explain it. Let the wrongness accumulate. The reader's discomfort at the end should have no clear source.