With this mode you can easily change the brightness levels in your map. Make a selection of sectors with LMB. Then hold RMB and drag up or down to increase or decrease the brightness levels of the selected sectors. The brightness changes to the nearest supported brightness level (according to the chosen game configuration), but you can hold Shift to override this behavior.
The order in which you select the sectors is important to the Make Brightness Gradient feature as it creates a gradient from the first selected sector to the last selected sector (the first and last are not modified). When you select the sectors, the order will be displayed with numbers in the sectors.