They're taking a comprehensive look at how imperative (React) or declarative-autotranslated-into-imperative (JSX) code for describing a View actually becomes that View (which is currently either (V)DOM, or some native UI toolkit with React Native).

This process occurs in browsers, libraries, and UI toolkits today; React gives you an alternative. This means that you can render in the client, render on the server, render wherever starting from the same code.