Every HTTP request in an ASP.NET Core application passes through a pipeline. This pipeline is built from middleware components. Each middleware can inspect the request, perform an operation and pass it to the next component.

Middleware ordering is very important. Components such as UseStaticFiles, UseRouting, UseAuthentication and UseAuthorization must be placed correctly. If authorization runs before authentication, the application may not know who the current user is.

Middleware can affect both the incoming request and the outgoing response. Logging, exception handling, security headers, localization and static file handling are all common middleware responsibilities.

Custom middleware can also be created. For example, you can measure request duration, block specific IP addresses or check for custom headers. Once added to the pipeline, the middleware runs for matching requests.

Understanding middleware helps you understand how ASP.NET Core applications actually work. Many issues related to authentication, localization and static files are solved by checking the middleware order.