Here's why:
* Noun: The core word "idea" is a noun, representing a thought or concept.
* Adjective: "Central" modifies the noun "idea", describing its importance or location.
* Phrase: Together, "central idea" functions as a unit expressing a single concept.
Therefore, "central idea" functions as a noun phrase within a sentence, often referring to the main point or theme of something like a story, essay, or speech.