React: What is React useSyncExternalStore?

1import { useSyncExternalStore } from "react";
2
3function useOnlineStatus() {
4  return useSyncExternalStore(
5    (callback) => {
6      window.addEventListener("online", callback);
7      window.addEventListener("offline", callback);
8      return () => {
9        window.removeEventListener("online", callback);
10        window.removeEventListener("offline", callback);
11      };
12    },
13    () => navigator.onLine,
14    () => true
15  );
16}