"Fiber" was the rewrite of React's internal render logic (primarily how it processes the component tree), and was released in 16.0. It's named after the data s

"Fiber" was the rewrite of React's internal render logic (primarily how it processes the component tree), and was released in 16.0. It's named after the data structure that is used to track individual pieces of work (roughly on a per-component basis). Fiber does specifically divide the overall update process into two general phases. "Render phase" involves walking the component tree, asking components what the desired UI should look like, and calculating the diff and necessary changes based on the previous requested output. Once that's been calculated, it is applied all at once in the "commit phase". Concurrent Mode is, loosely, additional modifications to that rendering logic to enable pausing partway through the "render phase" process, tracking one or more in-progress versions of render passes, prioritizing them based on the source of the requested update (user input vs network request, etc), and recalculating in-progress renders if another higher-priority render was pushed to the front and committed to the DOM first.