The root element is the root element of the React DOM tree.
It's the element passed to the rendering function ReactDOM.render where you will render your app or component.
In a app, you have at minimal a root rendering. But you may have several roots in web pages.
In a website that uses:
- only react, you would render only one root
- in another HTML application (php, ruby,…) with templating, you may render several roots (for instance to integrate react at several places in another HTML application (php, ruby,…)
See How to inject multi React components in a page? A Multiple Root demo