Rendering in React.js refers to the process of converting React components into DOM elements and displaying them on the screen. Rendering occurs when React elements are created, updated, or removed based on changes in state or props.

Here’s how rendering works in React.js:

1. Initial Rendering:
   When a React application is first loaded in the browser, the initial rendering process begins. React traverses the component hierarchy starting from the root component and generates a corresponding virtual DOM representation for each component.
2. Reconciliation:
   React performs a process called reconciliation to compare the current virtual DOM with the previous virtual DOM and identify the minimal set of changes needed to update the actual DOM. This process is efficient because React only updates the parts of the DOM that have changed, rather than re-rendering the entire tree.
3. Updating DOM:
   After reconciliation, React updates the actual DOM to reflect the changes identified in the virtual DOM. This involves adding, removing, or updating DOM elements as necessary to match the new virtual DOM representation.
4. Component Lifecycle:
   During the rendering process, React also invokes lifecycle methods of class components, such as componentDidMount, componentDidUpdate, and componentWillUnmount, allowing components to perform tasks like initializing state, fetching data, and cleaning up resources.
5. Re-rendering:
   Rendering in React is not a one-time process. Components can re-render multiple times in response to changes in state or props. When a component’s state or props change, React triggers a re-render of the component and its descendants, updating the virtual DOM and reconciling the changes with the actual DOM.
6. Conditional Rendering:
   React allows you to conditionally render components based on certain conditions using JavaScript expressions or conditional statements within JSX. Example:

   function MyComponent({ isLoggedIn }) {
       return (
           <div>
               {isLoggedIn ? <LoggedInComponent /> : <LoggedOutComponent />}
           </div>
       );
   }

Rendering in React is fast and efficient due to its virtual DOM implementation and reconciliation algorithm. By minimizing DOM manipulations and optimizing updates, React provides a smooth and responsive user experience, even for complex and dynamic applications.