It is the material when it comes to tying fine and natural dry flies or emergers. But also for nymphs, for example, to imitate the bubble during / just before emerging, CDC feathers are very well suited.
CDC stands for Cul de Canard, which means "butt of the duck". And that is exactly where the feathers come from. The material has a high fat content and can bind a lot of air, so that the feathers have an enormous float ability. And that's exactly why it is so popular among fly tying, because it requires little or no fat to make the dry flies float.
A package contains 100 CDC feathers.