Here's why:
* It describes a noun: You can say "a pretty flower," "a pretty dress," or "a pretty picture." In these cases, "pretty" describes the qualities of the nouns "flower," "dress," and "picture."
* It doesn't act as a verb or an adverb: It doesn't express an action (like a verb) or modify a verb, adjective, or other adverb (like an adverb).
Let me know if you have any other words you'd like to classify!