Linux is an operating system, same category as Windows and macOS but built with a different philosophy. It's free, open source, and designed for servers first. Launch a VM on AWS, Azure, or Google Cloud and Linux is the default. Docker, Kubernetes, most CI runners, most production databases, all Linux.
That's why every DevOps job description says "comfortable in Linux." You don't need to be a kernel developer. You need to sit at a Linux machine, look around, read a config, restart a service, and get out. That's the skill.
The good news: the commands you'll use day to day number in the dozens, not thousands. You'll meet most of them in this skill, one at a time.
Linux isn't one thing, it's a family. Ubuntu, Debian, CentOS, Red Hat, Amazon Linux, Alpine, all called distributions. They mostly behave the same. This course uses Ubuntu, which is what most cloud VMs default to.
You interact with Linux through the shell, a text program that reads what you type and prints the result. No mouse. On a server, the shell IS the interface.
Next up, meeting your first shell.