Here's why:
* It modifies a verb: Adverbs often describe how, when, where, or to what extent an action is performed. For example, you could say "Ella habla honestamente" (She speaks honestly).
* It ends in "-mente": Many Spanish adverbs are formed by adding "-mente" to the feminine form of an adjective. In this case, "honesto" (honest) becomes "honesta" and then "honestamente".