React: What is React Suspense boundaries?

1function App() {
2  return (
3    <Suspense fallback={<PageLoader />}>
4      <Header />
5      <Suspense fallback={<ContentLoader />}>
6        <MainContent />
7      </Suspense>
8      <Suspense fallback={<SidebarLoader />}>
9        <Sidebar />
10      </Suspense>
11    </Suspense>
12  );
13}