Fastlane is a tool used for automating the deployment process of mobile applications. It provides a set of built-in commands called actions that don't require installing any plugins or third-party tools. These actions can be used to manage app dependencies, build, test, and deploy iOS and Android apps with various options. Fastlane also offers actions for code signing, beta releases, production releases, screenshots, source code management, notifications, and integration with App Store Connect. By using these actions in a Fastfile, developers can simplify their deployment processes and speed up the build and release cycles of their mobile applications.