Module bundlers are tools that take multiple files and combine them into a single file or a few files. This process helps optimize web applications by reducing the number of requests a browser makes to load a page. Popular module bundlers include Webpack, Rollup, and Parcel, each offering unique features for managing dependencies and optimizing code. These bundlers work by analyzing the code and its dependencies, allowing developers to write modular code while still delivering efficient applications. They can also handle tasks like minification, transpilation, and code splitting, making it easier to manage complex projects and improve performance.