Here's why:
* It modifies an adjective or another adverb: It describes the degree or extent of something. For example: "The house is relatively small." (modifies "small") "He ran relatively fast." (modifies "fast")
* It answers the question "to what extent?": "How small is the house?" - "Relatively small." "How fast did he run?" - "Relatively fast."
Let me know if you have any other words you'd like to know the part of speech for!