What is carding of yarn?

Since one cannot card yarn, I assume what you're talking about is yarn spun from wool that has been carded, as opposed to combed or simply not carded at all. Carding is the process of brushing the wool prior to spinning to remove debris and tangles and to orient the individual hairs in more or less the same direction. Carded fibers are typically spun into woolen yarns (which are soft and fluffy) and combed fibers are typically spun into worsted yarns (which are smoother and harder).