Appium is a free and open-source test automation framework for mobile testing that can use any language supported by Selenium WebDriver. It allows users to write tests against iOS and Android platforms using the same API, enabling code reuse between test suites. The Appium server communicates with devices and test scripts over a standardized JSON protocol, making it easy to run on separate machines. With Appium, users can automate native, web, and hybrid mobile apps, and test on real devices, simulators, or emulators. The framework is designed for automated testing, taking advantage of developer skills, and supports various languages and platforms, including Safari on iOS and Chrome on Android.