Here's why:
* Noun: It names a concept: the main point or message of a piece of writing or speech.
* Phrase: It consists of multiple words working together to express a single idea.
Think of it like this:
* The central idea of the story is love. ("Central idea" is the subject of the sentence, functioning as a noun.)
* What is the central idea of this poem? ("Central idea" is the object of the preposition "of," acting as a noun.)
Let me know if you have any other questions about parts of speech!