Simplified explanation (shut up, Richard Stallman) : It's an operating system. In the same way a PC (or any other computer) has Windows (or OsX/MacOS, android, et.al.) as the connecting layer between the hardware and individual programs.

Linux can serve this role as well, and as it's free and open source, it's particularly popular among the tech and privacy enthusiasts here on Lemmy. Some use it for everything, some use it alongside windows (dualbooting), and some use it in a professional setting.

Linux is especially common on servers, but it's also gaining a lot of ground in the desktop space as well. Since Linux is free to distribute, many have over the years made lots of nux distributions specifically tailored towards particular use cases.

Source: Linux user of 25ish years.

Tip: The Linux mint installer can boot into a fully fledged Linux without touching anything on your hard-drive, so if you want to check it out without installing, you can.