The App Router: page.js and layout.js
Run the dev server and learn the two special files that define every Next.js route: page.js for a route's UI and layout.js for shared structure.
Start the development server
Time to see your app live. In the terminal, inside your project folder, run npm run dev. This starts the development server — a local web server (powered by Node) that compiles your code on the fly and serves it at http://localhost:3000.
Open that address in your browser and you'll see your home page. Now change the text in app/page.js, save, and watch the browser update instantly without a refresh. That live updating is called Fast Refresh, and it's one of the biggest reasons developers love Next.js.
Keep the dev server running in its terminal while you work. To stop it, press Ctrl + C in that terminal. To start again, run npm run dev. If port 3000 is busy, Next.js will offer the next free port (like 3001).