The HK-38 Lens Hood for AF-S NIKKOR 800mm f/5.6E FL ED VR Lens from Nikon is a bayonet style lens hood built specifically for the 800mm f/5.6 telephoto lens. It slips onto the end of the lens and is secured with a small thumb screw.
Lens hoods are primarily designed to prevent unwanted stray light from entering the lens and causing flare. In addition, since the end of the lens is extended, you receive the added benefit of some protection for the lens from accidental impact and the elements.
Dedicated lens hoods are designed to match the specific focal length of their lens: long enough to block stray light without extending into the field of view.