With a softcover, you have the option to have both the outside (the side you see when the book is closed) and the inside (the side you see when the book is open) of the cover printed.
With single-sided printing, only the outside is printed and the inside remains white. With double-sided printing, both the outside and the inside of the cover are printed.
This is separate from the exterior front and back of the book. These always form part of the cover and are printed as standard, if desired.