Mocha.js is a popular JavaScript testing framework that runs on Node.js and in the browser, making it easy to write and run tests for your applications. It supports both synchronous and asynchronous testing, and provides a simple, flexible interface for organizing tests using the describe and it functions. Mocha is highly extensible, allowing integration with various assertion libraries and plugins. It also includes features like before and after hooks to set up and tear down test conditions, and it offers a variety of reporters to display test results clearly. Mocha helps ensure your JavaScript code is reliable and maintainable by facilitating thorough testing.