Middleware functions are functions that have access to the request object (req), response object (res), and the next middleware function in the application's request-response cycle. Middleware can execute code, modify req/res objects, end the request-response cycle, or call the next middleware.

// Custom middleware
app.use((req, res, next) => {
  console.log('Time:', Date.now());
  next(); // Call next to continue to the next middleware
});