Here's why:
* Noun: A word that names a person, place, thing, or idea. "Idea" is the noun here.
* Phrase: A group of words that acts as a single unit of meaning, but doesn't have a subject and verb.
* Central: An adjective modifying the noun "idea."
Together, "central idea" refers to the main point or theme of a piece of writing, speech, or other communication.