Socket.io is a JavaScript library that enables real-time, bidirectional and event-based communication between the web browser and the server. It is widely used for building real-time web applications such as chat applications, real-time gaming, collaborative tools, and more.

To use Socket.io in your Express.js application, you first need to install the socket.io library using npm:

npm install socket.io

Once installed, you can create a new instance of the Socket.io object in your Express.js application:

const app = require("express")();
const http = require("http").Server(app);
const io = require("socket.io")(http);

The io object is a server-side component of Socket.io that handles the real-time communication between the server and the client.

To handle real-time events, you can listen to events on the io object, and emit events from the client-side using the socket object:

io.on("connection", (socket) => {
  console.log("A user connected");
  socket.on("disconnect", () => {
    console.log("User disconnected");
  });
  socket.on("chat message", (msg) => {
    console.log("Message: " + msg);
    io.emit("chat message", msg);
  });
});

In the above example, we listen to the 'connection' event on the io object, which is emitted when a new client connects to the server. Inside the callback function, we log a message to the console and set up event listeners for the 'disconnect' and 'chat message' events.

When the 'chat message' event is emitted from the client-side, we log the message to the console and emit the same event to all connected clients using the io.emit() method.

Socket.io also provides a number of other useful features, such as namespaces, rooms, and broadcasting. These can be used to organize your real-time application and manage communication between different groups of clients.

Overall, Socket.io is a powerful and easy-to-use library for building real-time web applications with Express.js.