Virtual DOM (VDOM) in React.js is a concept where a virtual copy of the actual DOM is kept in the memory. It is a lightweight copy of the actual DOM. React.js uses this Virtual DOM to increase the performance and efficiency of the application.
When the state of an object changes, React.js creates a new Virtual DOM and compares it with the old one. This comparison helps in identifying the updated elements in the actual DOM.
Instead of changing the whole DOM each time a change occurs, React.js only updates the necessary parts of the DOM that have been changed. This leads to a significant increase in performance and provides a smoother user experience.