Here's why:
* Adverbs describe verbs, adjectives, or other adverbs. They tell us *how*, *when*, *where*, or *to what extent* something is done.
* Adjectives describe nouns.
In a sentence, "brightly" would modify a verb, like this:
* The sun shines brightly. ("Brightly" describes *how* the sun shines)
Let me know if you have any other word type questions!