What does the originality score in the Quality review mean? Is it a plagiarism check?

No. The originality score is an AI opinion on how fresh the writing is: whether the ideas, examples and phrasing feel generic or distinctive. It does not compare your text with other books or with the internet, so it is not a plagiarism check and cannot prove or disprove copying. First drafts often score between 3 and 5 out of 10; that is normal and says the text could be more specific, not that anything was copied. Use the concerns and recommendations it lists as editing advice.