A main idea is the central idea which a story is centered on and which the <em>author </em>wants the reader to learn or take away from.
With this in mind, a supporting detail is used to validate a claim to prove that a claim is indeed factual and can be verified.
For example, bringing statistical data to prove that for example, blacks have the <em>most population</em> in America to support a claim which states that.
Without bringing accurate data or information to validate the claim, the claim would be <em>unverified</em>. This shows that supporting details are very important when making use of argumentative essays or when a person is trying to persuade or convince.
Please note that your question is incomplete so I gave you a general overview to help you
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