An adjective modifies (describes / distinguishes) nouns and pronouns. In other words, adjectives change nouns or pronouns in some way. So movie is a noun, and a scary movie has been changed by the adjective scary.
It’s important to remember, too, adjectives, as in the case of a scary movie, give you a way to inject your point of view into your writing. You might also describe a loveable book, a beautiful dress, or an ominous sky. There’s a certain amount of subjectivity, of course, in all of these words, so you’ll want to work to keep your audience in mind when choosing your adjectives and do your best to make sure your adjectives (or descriptors) are specific, concrete, and will make sense to both you and your audience.