Express.js is a web application framework for Node.js that is designed to make creating web applications easier. It handles requests by providing a series of middleware functions that are called in a sequence based on the request.
For example, when a client sends a request to the server, Express.js will first check for any authentication or authorization middleware functions that need to be called. If those pass, then the request is routed to the appropriate controller, which handles the logic for the request. Finally, the response is sent back to the client.