The Black Star is the highest award given by the Hairy Hooligan Tribe to it's warriors for outstanding bravery in the field of combat. The Black Star is a medal and is worn around one's neck.
As shown in How to Be a Pirate, Snotlout and many other Hooligans won Black Stars for their prowess during the Battle of the Lucky Thirteen.
During the events of How to Betray a Dragon's Hero, Snotlout is later seen wearing and holding his Black Star, and it is used as a reminder to him of a time before he betrayed his family and tribe. Before he flies off on the Windwalker, Snotlout gives Hiccup his Black Star. In the epilogue Hiccup states that Snotlout and the Black Star are a part of his kingship. Hiccup also states that he wears Snotlout's Black Star with pride, as Snotlout was a hero in the end. Over the decades since he obtained the Black Star, Hiccup says that the black layer covering the Black Star faded away, and now it is only gold.