Node.js uses the CommonJS module system to allow you to organize code into separate files, or modules, and reuse them throughout your application. The core of this system is the require function, which lets you load both built-in Node.js modules and your own custom modules. When you use require, Node.js reads the specified file, executes its code, and returns the value of module.exports from that file. This helps you keep your codebase modular, maintainable, and easier to test.

The require function works synchronously, meaning it will block further code execution until the module is loaded. You can use it to import modules by providing either the name of a built-in module or a relative path to a custom module. Built-in modules, such as fs for file system operations, come bundled with Node.js and do not require installation. Custom modules are files you create in your project, usually ending with .js.