Think of +100 as 100% The 255 that he mentioned is the maximum RGB value for Red 255, Blue 255, Green 255
Since the saturation and lightness values of the hue/saturation go from -100 to +100 you can actually apply it more than once if needed. (Until internally you hit the 255 values of RGB that is)
Also at the bottom is a gradient selector, even though in the drop down you choose reds, this selector can be adjusted to include or exclude neighboring colors.