Linux is a computer operating system.
Than again when most people talk about “Linux” they talk not only about the operating system but also all the free software that usually comes with it. (it would be like talking about Windows, Office, and a bunch of other programs as just “Windows” if they were bundled together, but unlike Linux, they usually aren't.)
This OS and software has been lovingly designed developed and debugged by a massive community of dedicated computer professionals and enthusiasts wanting to have something better that the commercial companies either cannot seem to do or wish to sell similar for as much as they can get (again and again).
Much of this software is free, free to copy, free to use in whatever way you want, free to modify, and… restricted from people trying to make these community works non-free.
If you do a lot of computer work and get tired and frustrated by commercial marketing and development, you might find Linux a breath of fresh air, things make a lot more sense when the code, development, and community are open.
Over half of the Internet runs on Linux Servers, not just because it's cheap, but because it is reliable.