That symbol marks someone as a Commander in WvW and PvE. It is obtained by purchasing the Commander's Compendium for 100 gold from a trainer.
As a commander, you are given access to a special set of tools for use in WvW/PvE, from the Wiki:
- /squadinfo will show the names of players in the squad and the amount of supply they are carrying. This command only takes squad
members into account.
- /supplyinfo will show the names of adjacent allies and the amount of supply they are carrying. This command is not limited to
squad members, but its radius is very small.
- ALT+Mouse1 opens a context menu for adding special order waypoints, to mark locations for attacks, defense, and
rally points. These special waypoints are visible to squad members.
The symbol also appears on the in-game map, making it much easier for other players to spot, in most cases, the location of the zerg for their team.