From the wiki: http://wiki.guildwars2.com/wiki/Adventure
“Adventures are a new feature in Guild Wars 2: Heart of Thorns, set to challenge level 80 players. Adventures are indicated on the map and in the game world, as illustrated to the right. (Picture below)
Every adventure is performance-based and repeatable, and instance the player. Adventures have accompanying leaderboards for friends and guild members, allowing players to compare and compete with each other.
Adventures are closely tied to masteries, how and how much haven’t been revealed yet.
When the player accepts an adventure, a new user interface element shows the player what to do. Each adventure has some tension, and is said not only to be time constraints.
Rewards
Players will be rewarded for meeting set tiers. Players will also be rewarded for placing on the leaderboard."
