"the Explorer" is a title awarded for completing  [Universal Explorer], for which you must completely explore all of the world maps for all continents in the game (Eastern Kingdoms, Kalimdor, Outland, Northrend, the areas revealed by the Cataclysm, Pandaria, Draenor, and the Broken Isles).

The title comes after the player's name, in the following format:

<name> the Explorer

In addition to the title, players will receive the  [Tabard of the Explorer] in the mail, upon completion of the exploration of Northrend.

Due to the terrain of Northrend, the achievement is not possible without [Cold Weather Flying], or some corpse running to get on to ledges that cannot be accessed by walking.

The Explorer Title and Tabard