It’s an operating system that originated as a clone of the ancient UNIX – much like Windows originated as MS-DOS. Linux is popular because the source code is licensed as free software (i.e. it’s in the cultural commons and free to be used and changed by anyone without fearing lawsuits). The fact that it’s free is why there are so many different operating systems built on top of it: Ubuntu, Fedora, etc. Linux is often used on servers, such as at Google, Microsoft, Amazon etc. However it’s also used on desktop PCs to a lesser extent. Because the source code is open to inspect, it’s considered more trustworthy than Windows/MacOS. This is also why https://www.privacytools.io/operating-systems/ recommends it.