A grid of fifteen circles undulates vertically and changes radius in a continuous sine-wave pattern, creating a flowing wave effect across the screen. Running on a timer that updates every 20 milliseconds, the animation redraws each circle's y-coordinate and radius based on a sine function. The code uses D3 v6’s `select`, `range`, and data joins with `join('circle')` to bind and update SVG circle elements, while the viewport-sized SVG fills the browser window.