Here's why:
* Adverbs modify verbs, adjectives, or other adverbs, providing more information about how, when, where, or to what extent something happens.
* In a sentence, "cautiously" would tell us how someone is doing something.
For example:
* "She walked cautiously through the dark forest." (modifies the verb "walked")
* "He was cautiously optimistic about the future." (modifies the adjective "optimistic")