Server-side rendering (SSR) is a technique that allows web applications to generate HTML on the server and send it to the client, rather than relying on client-side JavaScript to render the application. SSR can improve the performance and accessibility of web applications by reducing the time it takes for the initial page to load, and by ensuring that the application is visible to users who have JavaScript disabled or slow network connections.

Next.js is a popular React framework that provides built-in support for server-side rendering. Next.js allows developers to build server-rendered React applications with minimal setup and configuration, and provides a number of features and optimizations to improve the performance and user experience of server-rendered applications.

To build a server-rendered React application with Next.js, you can start by creating a new Next.js project using the `create-next-app` command in your terminal:

npx create-next-app my-app

Once your project is set up, you can create React components and pages as you would in a standard React application. However, in a server-rendered application, you must also define a `getServerSideProps` function for each page that needs to be rendered on the server. The `getServerSideProps` function is responsible for fetching any data that the page needs to render, and returning it as props to the page component.

Here is an example of a simple server-rendered page in Next.js:

import React from 'react';

function HomePage({ data }) {
  return (
    

      
Hello, world!
      
{data}
    
  );
}

export async function getServerSideProps() {
  const res = await fetch('https://api.example.com/data');
  const data = await res.json();

  return {
    props: {
      data
    }
  };
}

export default HomePage;

In this example, the `HomePage` component is a simple React component that displays a greeting and some data. The `getServerSideProps` function fetches data from an external API and returns it as props to the `HomePage` component. When the page is rendered on the server, the `getServerSideProps` function is called to fetch the data, and the resulting HTML is sent to the client.

Overall, Next.js provides a powerful and flexible framework for building server-rendered React applications. By using Next.js, you can improve the performance and accessibility of your web applications, and provide a better user experience for all users, regardless of their device or network connection.