Your shell has these "hidden variables" that can change how other programs work. In this exercise I first print out my environment, just dumping it to the screen. Then I do it again but pipe it through grep to find only the variables with my username in them. Finally, I set an environment variable TESTING to "1 2 3".
You may not run into this too often, but sometimes to get something configured you have to change these. A good example is the PATH variable, which determines the search order for other commands. Changing your PATH is going to be one of your exercises.
For the price of most other course's PDFs only, you can get the full PDF for this class and 2 videos demonstrating the whole book for both Unix/OSX Terminal and Windows PowerShell. The course is self-paced so you can go through it any time you want, as many times as you want.