Most importantly, it shows you the scale of certain departments when it comes to employment.
This rectangle represents all the employees of the federal government. It's divided into smaller boxes that correspond to individual departments. Those boxes are sized proportionally to the number of people who work in them. The red portion are furloughed employees. The blue portion are people exempt from furlough and still on the job. The different shades of blue refer to why they're still on the job. 
So, for instance, NASA — a smaller box on the lower right — has 97% of its employees furloughed. 
Finally, the government shutdown in one chart:

As the Washington Post's Brad Plumer noted on Twitter, the Department of Defense indicated over the weekend they'd be recalling many of their workers, so this chart is subject to get a bit bluer the left side.