In the example above I was wondering what was taking so much space in the folder "c:/files/epicent".
The diagram tells me that the "epicent" folder is 1.8 Gb (the blue rectangle to the left). Epicent contains several subfolders (the rectangles to the right), where the biggest is "misc" (1 Gb). The biggest subfolders under misc are "demo cd" and "sample-data".
Since the rectangle sizes correspond to the relative sizes of the folders, smaller folders are hidden, which makes it easy to identify the real culprits eating up your disk space. You can then use the popup menu to delete folders that you don't need.