I don't even know where to start tbh. I remember using ms dos as a kid and installing games off floppy disks. I knew the commands to execute programs or read setups before executing but other than those two things I didn't know what I would do further than that.
Discussion
Also kind of depends on what command line you're trying to learn. Linux has its own quirks versus Windows and Mac.
The basics are things like folder navigation, manipulation, and text editing. Then you can dive deeper on data manipulation and whether or not you're interacting with other programs or just the core operating system itself.
I hope you verify signatures