Here's why:
* Adverb: It modifies a verb, adjective, or another adverb. For example, "Where did you go?" (modifies the verb "go")
* Relative: It introduces a dependent clause that modifies a noun or pronoun. For example, "This is the place where I met you." ("Where" modifies "place" and introduces a clause that tells us more about it).
Let me know if you'd like to know more about parts of speech!