The treemap shows the whole contents of the directory tree straight away.
It represents each file as a colored rectangle, the area of which is proportional to the file's size.The rectangles are arranged in such a way, that directories again make up rectangles, which contain all their files and subdirectories. So their area is proportional to the size of the subtrees.
The color of a rectangle indicates the type of the file, as shown in the extension list.
The cushion shading additionally brings out the directory structure.
You effortlessly see the big rectangles, that is the big files, even if they are hidden deeply in subdirectories in the directory tree. Click on them: the directory list opens the path to the file, and you can read its location and attributes, and - if you like - apply a cleanup action on it, e.g. delete the file.
Navigate in the directory list: the respective selection is highlighted with a colored frame. In this way gain an impression of the proportions: How big, for example, is C:\windows compared with C:\program files?
The context menu options "Select Parent" and "Re-select Child" are also useful for the navigation.
Zoom. The option "Zoom in" enlarges the treemap, so that a subtree is displayed full size. As it were you make a step towards the selected item. The directory list show the root of this subtree with a blue frame. By "Zoom out" you step back, and the parent element is shown again.