GNU is the name of a computer operating system. The name is short for GNU's Not Unix. Richard Stallman leads the GNU Project. The popular Linux operating systems made using Linux kernel include many GNU tools too. So, many projects and developers call the Linux-based operating systems GNU/Linux.
The GNU project was started by Richard Stallman in 1983. His wanted to create a computer system that was all free and open source software. Users could change, share and publish new work based on GNU. He and a group of developers started by creating copies of each piece of Unix software. The rest of GNU was the kernel, called the GNU Hurd, which is not yet finished. The more popular Linux kernel is used instead.