A Kabuto is a Samurai helmet. As demonstrated by the show, it is strong enough to survive heavy blows, even from viking axes. The curved design makes a weapon slide off of it and its hard metal makes it easy for the impact to be dispersed. Some Kabutos are decorative, the ones in the show sometimes had an image of a crescent moon.
The Japanese during the Edo period had the Chonmage as the most common hairstyle for Samurai and similar social elites. The idea with this hairstyle was to create a baldspot where the Kabuto could rest on the head more comfortably.