Here's why:
* It modifies a verb, adjective, or another adverb: It tells us *how* something is done or to what extent something is true. For example:
* "The movie was incredibly funny." (modifies the adjective "funny")
* "He ran incredibly fast." (modifies the adverb "fast")
* "She smiled incredibly brightly." (modifies the verb "smiled")
Let me know if you have any other word types you'd like to explore!