The reflectivity map is the average amplitude of all images.
The amplitude is measured in volts.
Values there are the output of a normalization.
Usually amplitude is plotted between 0 and 3, you can change the upper value in the advanced parameters http://sarproz.com/manual/adv_param.html
The actual maximum value depends on the strongest scatterer in the scene.
To see the full range of values, you can plot an histogram here http://sarproz.com/manual/isto.html