Here's why:
* It describes a noun: You would say "an independent thinker" or "an independent country." It modifies the nouns "thinker" and "country" to provide more information about them.
* It doesn't act as a verb or a noun itself: It doesn't describe an action (like a verb) or refer to a person, place, or thing (like a noun).
Let me know if you have any other words you'd like to figure out!