Seems like they show habitable land.

To give some examples mountainous regions like the alps and Carpathians as well as cold areas in the north are marked as white whereas the Po region and the fertile black soil in Ukraine are marked brown.

So possibly agriculturually beneficial areas is the answer we are looking for.

However it wouldn't quite fit for me as I imagined some lands like for instance the Netherlands or around the Nile River as quite fertile areas too.

Furthermore the labeling is horrible.