The command line is used to, um, give commands to your computer. It has a bit of a learning curve, but once you know a few shortcuts and common commands it is one of the most powerful tools to get what you want out of your machine..
Note, the CLI is also called shell or terminal. They are the same thing. Well, almost, but for now it will do.
- Shell Fu - Learn 'em shourtcuts and basic commands. Interactive, web course.
- The Command Line Crash Course - Read the book online.
The basics are probably enough. You should make sure that you have mastered touch typing and learned some ruby before continuing. If you're interested, however, now that you know the basics, you can move on to the really powerful stuff: