Adjectives are words that primarily modify nouns. They may also modify pronouns or other words that are functioning as nouns or noun phrases.
The fluffy dog curled up contentedly next to the couch.
The adjective fluffy here modifies the noun dog. Adjectives that precede or follow the word they modify in this way are known more specifically as attributive adjectives.
The old movie was really boring.
The adjective boring describes the subject movie. In this case, boring is considered a predicative adjective because it is located in the predicate of the sentence and is connected to the subject via the linking verb was.