Welcome to the forum!
CDC is short for cul-de-canard. These are downy feathers found around the preen gland on the backs of waterfowl. Watch ducks on the water and you'll see them rubbing their bills on the preen-gland (located on the back just in front of the tail) - hence the term "preening". A naural oil is exuded by the gland which is then distributed through the plumage to keep themseves "waterproof". CDC feathers have a natural bouyancy and great mobility. It is readilly available from any fly-shop with a decent selection of fly tying materials.
When sight fishing, look over your shoulder from time to time, you never know who's behind you