A cuttlefish bone is, unsurprisingly enough, the bone of a cuttlefish. (The cuttlefish is actually not a fish at all, but a cephalopod. A cephalopod is a mollusk with an internal shell--like a squid or octopus.) A cuttlefish bone should be provided in the cage, tank, or living area of any bird or reptile, especially turtles which require a great deal of calcium.
The soft bone of the cuttlefish is an especially good source of calcium for birds and reptiles, as it is usually very dry. Take care to place the softer, drier side of the bone up (when laying it down) or in (when mounting it on a birdcage), so that your pet can more easily access the softer bits, which is the whole point of having the damn thing hanging out in your pet's ecosystem anyways.