A React application renders a single SVG circle that follows the mouse cursor across the viewport. The circle's center coordinates are updated via the `useState` hook and the `setMousePosition` callback, which reads the `clientX` and `clientY` properties from the `onMouseMove` event on the SVG element. Using React's declarative model, the circle is redrawn at the new position whenever the mouse moves, with its radius fixed at 30 pixels. The code imports React and ReactDOM to mount the app into the DOM.