What is GNU in operating system?

GNU is a Free Software project which is essential to the Linux operating system. It is a project that provides and distributes a lot of the software which is essential to making a fully-functional operating system distribution based upon the Linux kernel. For example, the Linux tools/commands that allow you to enter and run commands, copy files from one directory to another, compress and uncompress files, search for strings within text files, and mount and unmount filesystems are all GNU-provided tools. In short, GNU is a project which provides a major portion of the base Linux operating system - besides Linux itself which is the kernel and is provided by a separate project. It does not however provide the entire rest of the operating system; primarily just the type of basic tools mentioned above (although a very lot of them). Some GNU software has also been ported to run on other operating systems also. Some people refer to the Linux operating system as GNU/Linux to reflect the major contribution which GNU provides to that operating system.