If you’re unfamiliar with Linux, you’re not alone. Most home computer users use either the Windows or Mac operating system (OS).
Linux (or more technically “Linux/GNU”) is actually hundreds, if not thousands, of available operating systems, that much like Windows or MacOS are capable of storing files and running programs to interact with those files.
Linux is Open Source which is somewhat like a copyright agreement, except you can think of it as a copy-left, where the agreement is actually that you’re allowed to edit, and share any of the programming as long as you’re not selling it.
So, because anyone with the know-how and interest can take the programming of Linux, make changes and republish those changes as a unique operating system, you can see how a few main OS’s can split into some large families of related operating systems that are all somewhat unique.