They mean that after the previous steps, the intensities may not span the full allowable range (usually 0 to 255), so you might need to readjust. But your image processing software should already do that when you apply an unsharp mask.
Bottom line: This filter subtracts a blurred version of an image from itself. Hence, it is a high pass filter, meaning that it throws out long range variations so that short range ones are emphasised. It may appear that it brings out more detail, but it does so by throwing away information. It mutes the bass so you can better hear the lead.
