GNU Toolchain is a collection of software development tools that enable developers to create applications for various platforms, including operating systems and embedded systems. It includes tools such as compilers, linkers, debuggers, and libraries that help developers in the process of building and optimizing software for different environments. GNU Toolchain is open-source software and is maintained by the Free Software Foundation (FSF). Proficiency in GNU Toolchain is important for developers who work on low-level programming and embedded systems.