Appium is an open-source tool designed for automating mobile applications. It supports both native and hybrid apps on platforms like iOS and Android. Developers can write tests using various programming languages, including Java, Python, and JavaScript, making it versatile for different tech stacks. One of the key features of Appium is its ability to work without modifying the app being tested. This means that developers can test their applications in a real-world environment, ensuring that the user experience remains intact. Appium also integrates well with other testing frameworks, enhancing its functionality and ease of use.