A central idea can also be called a main idea.
It is not just a topic or subject.
It will name the topic and make a statement about that topic.
A central idea will be a complete sentence.
It will be a declarative sentence, never a question.
It will be specific to the text.
It will not be a theme. = It will not be a universal message.
Examples and Non-Examples:
Jim Thorpe may have been the greatest athlete of all time.
Jim Thorpe was an outstanding athlete in many sports.
Jim Thorpe [This is just a topic. A central idea will be a complete sentence.]
The greatness of Jim Thorpe [This is just a topic. A central idea will be a complete sentence.]
This paragraph tells about what a great athlete Jim Thorpe was. [This is just another way of stating the topic.]
A person can excel in many ways. [This is a theme. It is not specific to Jim Thorpe.]
Each supporting detail needs to provide support or explanation for the central idea.
With Central Idea, we call the sub-ideas and evidence supporting details.
More examples of central ideas from nonfiction books:
One of the core aspects of the race to build the atomic bomb was spying on one another.