What are these columns and how can I make use of them ?
They're all empty. Using WMP 11 under Windows 7.
They'll display the information that's in the "Custom1" and "Custom2" tags in your mp3 files (assuming you've put some data on those tags of course).
See this Super User question for some more information around this area.
Not all music apps will read or write these. Media Monkey lists 5 custom fields on it's properties dialog: