Here's why:
* It modifies a verb, adjective, or another adverb: It describes how something is done (verb), how something is (adjective), or to what extent (another adverb).
* Example:
* "He completely forgot about the meeting." (modifies the verb "forgot")
* "The room was completely empty." (modifies the adjective "empty")
* "She walked completely out of the room." (modifies the adverb "out")
Let me know if you'd like more examples!