Middleware is quite simply a bridge between database and application. In the context of ASP.NET Core, middleware is also a bridge between two components. It is middleware that decides how to respond to HTTP requests in ASP.NET Core. ASP.NET app experts are often looking at Middleware to handle requests or to be a bridge.
Middleware also controls how the application looks when there is an error, and it is a key link in authenticating and authorizing users to perform specific actions. In other words, middleware are software components assembled into an application to handle request and response to performing user actions before/after another component is invoked.