Reconciliation: React uses reconciliation to efficiently update the DOM when a component's state or props change.

Virtual DOM: React maintains a lightweight copy of the real DOM, called the Virtual DOM. When a component updates, React generates a new Virtual DOM tree to represent the updated state.

Diffing Algorithm: React compares the old Virtual DOM tree with the new one using its diffing algorithm. It finds the minimum set of changes required to update the real DOM.

In Stack Reconciliation: The entire render process was synchronous and monolithic. React walked through the Virtual DOM tree and performed updates in a single, uninterrupted pass. Large updates could block the main thread, causing lag or unresponsiveness.

React fiber:

• React Fiber is the reconciliation algorithm and introduced in React 16.
• it is a re-architecture of React's rendering process.
• The goal was to enhance the rendering pipeline, improve responsiveness, and support new features like Concurrent Mode and Suspense.
• Fiber refers to a data structure that holds information about a component tree, enabling React to perform rendering work incrementally and asynchronously.

Key Concepts in React Fiber: The Fiber Node: Each element in the React tree (like components or DOM nodes) is represented by a Fiber node. Fiber nodes are linked to form a Fiber tree, which is the underlying structure React uses during rendering and reconciliation.

Two Phases of Rendering: React Fiber splits the rendering process into two distinct phases:

Render Phase (Reconciliation): The Fiber tree is built. React determines what updates are required (diffing the new and old trees). This phase is interruptible.

Commit Phase: React applies the changes (DOM updates, refs, and lifecycle methods). This phase is synchronous and cannot be paused.