The virtual DOM is a simplified abstraction of the real DOM. In React, instead of manually manipulating the real DOM, developers should instead manipulate the Virtual DOM. React would then compare the old Virtual DOM state with the new one, and calculate the most efficient way of manipulating the real DOM.

DOM manipulation is a heavy operation and humans often don't see the most efficient method of manipulating it. Therefore, having React automatically calculate the most efficient way makes updating the DOM much more efficient, and leads to a faster, more reactive UI.