The Zone Guide is a system that was added in Update 21 which lists achievements and other activities for each zone, showing you things you have yet to achieve in that zone. It appears under the Group & Activity Finder, but can also be reached via the map screen. The guide lists up to 4 Achievements in addition to the following:
Main Story Quests
Wayshrines
Delves
Group Delves
Points of Interest
Striking Locales
Set Stations
Mundus Stones
Public Dungeons
World Events (Dark Anchors, Abyssal Geysers)
World Bosses
Skyshards
Shalidor's Library Books
This info is also displayed in a summary on the left side of the Map screen.