Q: What is the "nap"?

A: All fur consists of two parts, or layers. There is a short, fuzzy layer called the "underfur" and longer, shiny hairs more sparsely spread out called the "guard hairs". The distance between the top of the underfur and the tip of the guard hairs is called the "nap". Nap is only relevant in regard to shorter hair furs such as mink.
Using mink as an example: The higher, or longer, the nap is, the harrier the coat will look. The shorter the nap, the more velvety the garment will appear.

http://www.dittrichfurs.com/dittrich_faq.htm

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Typically, (but far from always) female pelts have a lower nap (creating a more velvety appearance) which is more desirable in a finished garment.