In general, middleware is plumbing software that facilitates communication flow between two components. In a web application framework, middleware provides common services and capabilities to the application outside of the default framework.

In ASP.NET Core, middleware refers to the C# classes that manipulate an HTTP request when it comes in or an HTTP response when it’s on its way out. For EXAMPLE,

Generate an HTTP response for an INCOMING HTTP request
Intercept and make changes to an incoming HTTP request and pass it on to the next piece of middleware.
Intercept and make changes to an OUTGOING HTTP response, and pass it on to the next piece of middleware.

One of the most common use cases for middleware is to deal with concerns that affect your entire application. These aspects of your application need to occur with every request, regardless of the specific path in the request or the resource REQUESTED. These include things like logging, security, authentication, authorization, etc.

For example, logging middleware logs when a request comes in and passes it to another piece of middleware. Some other common middleware USES include database middleware, error handling middleware, and authentication/authorization middleware.

In each of these examples, the middleware receives a request, modifies it, and then passes it to the next middleware piece in the pipeline. Subsequent middleware uses the details added by the earlier middleware to handle the request in some way. The following diagram illustrates this.