Express is a routing and middleware web framework that has minimal functionality of its own: An Express application is essentially a series of middleware function calls.

Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. The next middleware function is commonly denoted by a variable named next.

An Express application can use the following types of middleware:

• Application-level middleware
• Router-level middleware
• Error-handling middleware
• Built-in middleware
• Third-party middleware

Source:

Using Express Middleware. expressjs.com/en/guide/using-middleware.html.