Here's why:
* It describes a noun: You can say "The smell was repulsive" or "The idea was repulsive." It modifies the noun "smell" or "idea" to tell us more about them.
* It answers the question "what kind?": The smell was *what kind*? Repulsive. The idea was *what kind*? Repulsive.
Let me know if you have any other words you want to identify!