In Express.js, middleware is a function that can access the request object (request), the response object (response), and the next middleware function in the application's request-response cycle. Middleware is widely used for executing code, modifying request and response objects, terminating the request-response cycle, and calling the next middleware in the middleware stack. Middleware can be layered and executed in sequence, allowing developers to execute middleware functions one after another in the order defined in the code. This mechanism enables developers to easily insert necessary functionality during request processing, such as error handling, logging, user authentication, and data validation.