Here's why:
* Verbs express actions or states of being. "Is" is a form of the verb "to be," indicating a state of being.
* Adjectives describe nouns or pronouns. They tell us what kind, how many, or which one.
For example:
* The cat is fluffy. ("Fluffy" is the adjective describing the cat.)
* The sky is blue. ("Blue" is the adjective describing the sky.)
In both sentences, "is" is the verb connecting the subject to the description.