Here's why:
* It modifies a verb: Adverbs tell us *how*, *when*, *where*, or *to what extent* something happens. For example: "She actively participated in the discussion." "Actively" tells us *how* she participated.
* It ends in "-ly": Many adverbs in English end in "-ly."
Let me know if you have any other words you'd like to know the part of speech for!