Next.js Middleware is a server-side feature that allows developers to inspect and modify incoming requests before they reach the application's route handler, enhancing functionalities such as authentication, redirection, and logging. This middleware operates using the Edge Runtime, which is optimized for speed and is globally distributed, ensuring faster response times by processing requests closer to users' locations. Middleware is particularly useful for tasks like language detection, route localization, cookie management, feature gating, logging, and rate limiting, but it is not suited for heavy computations or operations requiring persistent memory or database interactions. The guide provides practical examples for setting up and deploying middleware in various environments, highlighting how to leverage Next.js Middleware for tasks like CORS handling, conditional request management, and enhancing applications with Contentful for creating personalized digital experiences.