What is System Data on an iPhone's storage, and how much is normal?

Your iPhone's storage breaks down into many categories; System Data is often large but doesn't have a clear explanation.

Your iPhone&#39;s storage breaks down into many categories; System Data is often large but doesn&#39;t have a clear explanation.<p><img src="https://www.engadget.com/img/gallery/what-is-system-data-on-an-iphones-storage-and-how-much-is-normal/intro-1793803853.jpg" /></p>