You may have heard of React Scan for automatically detecting performance issues. It uses Bippy internally. From the README:

a react fiber is a “unit of execution.” this means react will do something based on the data in a fiber. each fiber either represents a composite (function/class component) or a host (dom element).

fibers are useful because they contain information about the react app (component props, state, contexts, etc.). a simplified version of a fiber looks roughly like this:

interface Fiber {
  // component type (function/class)
  type: any;

  child: Fiber | null;
  sibling: Fiber | null;

  // stateNode is the host fiber (e.g. DOM element)
  stateNode: Node | null;

  // parent fiber
  return: Fiber | null;

  // the previous or current version of the fiber
  alternate: Fiber | null;

  // saved props input
  memoizedProps: any;

  // state (useState, useReducer, useSES, etc.)
  memoizedState: any;

  // contexts (useContext)
  dependencies: Dependencies | null;

  // effects (useEffect, useLayoutEffect, etc.)
  updateQueue: any;
}

Further reading and watching:

• Why does React have “Rules of Hooks?”
• How does React re-render internally?
• Live visualization of a fiber tree