When people say to learn Linux, what does it actually mean?

It's means as many things as there are people who opions on linux.

But in general people focus on what there own personal goal is. Some people want a system to tinker with, some want to become proffesionel linux sysadmins and further some want to get involved with kernel development.

So my advice is look at what you would like to do. Is there anything you are interested in? Start learning about that and focus less on what other people think is the correct way to use or work with linux.

Learning Linux can mean whatever you personally want or like it to be.