In the context of React.js or other reactive frameworks, "re-render" refers to the process of updating the user interface in response to a change in state or props. When certain data in a React component changes, React re-invokes the render method of that component, creating a new virtual DOM representation. React then compares this new virtual DOM with the previous one to identify the differences (reconciliation) and updates only the parts of the actual DOM that need to change.

Re-rendering doesn't necessarily mean the entire DOM is rebuilt. React's virtual DOM diffing algorithm is efficient, and it aims to make the minimum necessary changes to reflect the updated state.

Understanding when and why re-renders occur is crucial for optimizing React applications. Unnecessary re-renders can impact performance, so React provides tools like the shouldComponentUpdate lifecycle method or the React.memo higher-order component to optimize and control when components should re-render.

Conclusion:

In summary, re-rendering is the process of updating the user interface to reflect changes in the underlying data or state of a React component. React's virtual DOM allows for efficient updates by selectively applying only the necessary changes to the actual DOM.