React: What is React 19 streaming SSR?

1import { renderToPipeableStream } from "react-dom/server";
2
3app.get("/", (req, res) => {
4  const { pipe, abort } = renderToPipeableStream(
5    <App url={req.url} />,
6    {
7      bootstrapScripts: ["/client.js"],
8      onShellReady() {
9        res.statusCode = 200;
10        res.setHeader("Content-type", "text/html");
11        pipe(res);
12      },
13      onShellError(error) {
14        res.statusCode = 500;
15        res.send("<h1>Error</h1>");
16      },
17    }
18  );
19});

1// loading.tsx
2export default function Loading() {
3  return <div>Loading...</div>;
4}
5
6// page.tsx
7export default async function Page() {
8  const data = await fetchData();
9  return <Content data={data} />;
10}