You can customize which columns are visible in the TreeView using this window. You simply click and drag an item in the window out to the headers in the tree. There you will see something like the following screen capture - showing the Green arrows that will show you where the column will dock to.
To remove a column - simply click on the header and drag it to the Customize window.
You can also move the columns around switching their positions. It should be noted that the program will remember which columns are dislayed, their widths and their positions each time Disk Triage starts.
When you are done - click on the 'X' to close the customize window.