High Explorer (aka High Explorer of the Guild) is a title of honor or rank in the Explorers' Guild. Several members of the guild have or had this rank. There is evidence that High Explorer was also originally the title for the leader of the guild. It appears that more than one member can hold the title at the same time in recent years.
Brann Bronzebeard shares the title of High Explorer along with the leader of the guild, Muninn Magellas. Brann Bronzebeard has signed his name as "High Explorer Brann Bronzebeard of the Explorer's Guild". It is unclear how he got this status. Perhaps the guild policies have changed allowing the rank to be held by more than one individual. Another possibility is that it represents foreshadowing, suggesting that Brann Bronzebeard has recently been promoted to High Explorer replacing Muninn Magellas, perhaps reflecting his status in Wrath of the Lich King. In Dark Factions, Brann only signs in his less formal "Brann Bronzebeard" signature.