A bundler is a tool that helps to simplify the process of managing and organizing JavaScript modules. It allows developers to combine multiple separate JavaScript files into one bundle file, which can then be included in an HTML page. This makes it easier to manage code, as well as reducing the number of requests the browser needs to make when loading a page. Bundlers also provide features like minification, transpilation, and tree-shaking, making it easier for developers to write modern JavaScript code and optimize their applications for performance.