Frightening things are scary or alarming. The sign of a really well-made horror film is that it's deeply frightening.
Ghost stories are frightening, and so are car accidents, creepy old houses, and news stories about climate change. All of these things are capable of scaring people, causing them fright or terror or dread. Frightening and the verb frighten come from the noun fright, with its Old English root fyrhtu, which means "fear, dread, trembling," and also "horrible sight."