Modules are a fundamental concept in Node.js that allow you to break your code into smaller, reusable pieces. A module is simply a file containing JavaScript code that is isolated from other files unless you explicitly share parts of it. This approach helps you organize your code, making it easier to maintain, debug, and reuse across different projects.

Using modules is important in Node.js because applications often grow large and complex. Without modules, all your code would live in a single file, making it difficult to manage. By splitting functionality into separate modules, you can focus on individual features or utilities, avoid naming conflicts, and encourage code sharing. Modules also enable you to use third-party libraries and share your own code with others.