This code sets up a route handler for the root...

This code sets up a route handler for the root URL of a web application using the Express.js framework. When a user navigates to the root of the application, the server will respond by executing the provided callback function (req, res) => { ... }.

The req parameter represents the HTTP request made by the client, while the res parameter represents the HTTP response that the server sends back to the client.

In this specific case, the callback function calls the send() method of the res object, which sends a string response of "Successful response." back to the client's browser. This indicates that the request was successful and the server has sent the expected response.