Perhaps the biggest change in React 16 is the change made to the internal reconciliation code. These changes don't impact the way that you interact with the React API. Instead, these changes were made to address some pain points that were preventing React from scaling up in certain situations. For example, one of the main concepts of this new architecture is that of fibers. Instead of rendering every component on the page in a run-to-compilation way, React renders fibers—smaller chunks of the page that can be prioritized and rendered asynchronously.
For a more in-depth look at this new architecture, these resources should be helpful:
