Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something is done.
* In a sentence, "briefly" would answer questions like "How long?" or "To what extent?".
For example:
* "She briefly explained the situation." (modifies the verb "explained")
* "He was briefly mentioned in the report." (modifies the verb "mentioned")
* "I was briefly surprised." (modifies the adjective "surprised")