The HB-66 Bayonet Lens Hood from Nikon is a black petal-style lens hood designed for the AF-S NIKKOR 18-35mm f/3.5-4.5 G lens.
Lens hoods are primarily intended to prevent unwanted stray light from entering the lens and causing flare. In addition, since the end of the lens is extended, you get the added benefit of protection from accidental impact and the elements.
The "petal" shape of this lens hood maximizes the amount of hood coverage for the minimum amount of weight and hood area. This dedicated lens hood is designed to match the specific focal length of its lens, deflecting stray light without extending into the frame.