Appium is a test automation tool that allows testing of mobile apps without modifying the app code. It can test both Android and iOS apps using various programming languages like Ruby, Python, Java and JavaScript. Appium works by mimicking user actions on an app and can be used for regression and acceptance testing. The document provides examples of using Appium to test the Liputan6 and Vidio mobile apps, checking that features like menus, search, and login functions work as expected.