The command line is a text interface for your computer. It's a program that takes in commands, which it passes on to the computer's operating system to run.
From the command line, you can navigate through files and folders on your computer, just as you would with Finder on Mac OS or Windows Explorer on Windows. The difference is that the command line is fully text-based.
The advantage of using the command line is its power. You can run programs, write scripts to automate common tasks, and combine simple commands to handle difficult tasks - making it an important programming tool.
This course is for unix-based systems such as Linux and Mac OS X. An appendix of all commands taught in this course is available here.