Here's why:
* It modifies a verb: It describes how an action is performed. For example, "The children ran excitedly to the park." The adverb "excitedly" tells us how the children ran.
* It ends in "-ly": Many adverbs are formed by adding "-ly" to adjectives. The adjective form of "excitedly" is "excited."
Let me know if you have any other grammar questions!