Viscosity is the ability of a liquid to flow. Water, for instance, has a low viscosity since it flows so easily. The viscosity of diesel fuel is important since it must be low enough that it flows easily through the injection system, while at the same time being high enough to lubricate the moving parts in the injection system. No. 2 diesel fuel has a higher viscosity than No. 1, which means it lubricates better, but does not flow as well. Because of this and its lower cetane rating, No. 2 diesel is not as satisfactory as No. 1 in extremely cold weather.
One more word on diesel fuels. No matter what you've heard elsewhere, don't thin diesel fuel with gasoline in cold weather. The lighter gasoline, which is more explosive, will cause rough running at the very least, and may cause extensive engine damage if enough is used.