React: What is React 19 ref callback cleanup?

1function TrackedElement() {
2  return (
3    <div
4      ref={(node) => {
5        if (node) {
6          // Setup
7          const observer = new ResizeObserver(handleResize);
8          observer.observe(node);
9
10          // Cleanup
11          return () => observer.disconnect();
12        }
13      }}
14    >
15      Content
16    </div>
17  );
18}