Here's why:
* Adverbs modify verbs, adjectives, and other adverbs.
* Verb modification: "He ran *quickly*." (modifies the verb "ran")
* Adjective modification: "She was *extremely* tired." (modifies the adjective "tired")
* Adverb modification: "He spoke *very* slowly." (modifies the adverb "slowly")
Let me know if you'd like more examples!