Adjectives are words that describe and modify nouns to provide more detail and paint a clearer picture for the reader. They typically answer questions about an item's kind, type, number, or amount. The document provides examples of adding adjectives like "tall", "young", "large", and "black" to modify nouns like "man", "dog", and "bird" in sentences. It encourages adding at least three adjectives to the sentence "The boy is throwing the ball" and being prepared to share the modified sentence and questions answered with the class.