Middleware is a layer that inspects or filters an HTTP request before it reaches a route or controller, and it can also act on the response on the way back. Laravel uses middleware for concerns such as authentication, authorization, CSRF protection, throttling, logging, and request transformation.

A middleware either passes the request deeper into the application by calling the next callback or returns a response early. Keeping these cross-cutting concerns in middleware prevents controllers from repeating the same checks on every action.