Here's why:
* It describes a noun: "Wealthy" tells us something about a person, place, or thing. For example: "The wealthy businessman donated millions to charity."
* It modifies a noun: "Wealthy" changes the meaning of the noun it's attached to. In the example above, "businessman" is the noun, and "wealthy" modifies it to tell us what kind of businessman he is.
Let me know if you'd like more examples!