QuestZone modifies the behaviour of the Quest Tracking window to only show quests with Points of Interest in the current zone.
Ever been levelling through a zone, and you picked up half a dozen quests at a hub, which filled your tracking frame half of which are for a dungeon. You could un-track them but then you might forget them when you get in the instance, and you'll have to untrack all the non-instance quests to see them on the tracker.
QuestZone will hide all the irrelevant quests when you enter an instance, or change zone. Enter Slave Pens? All you'll see on the tracker is Slave Pens quests. Exit to Zangarmarsh and all your Zangarmarsh quests will appear and Slave Pens quests disappear.
If you want to you can override the behaviour and always show a quest if it's available, and QuestZone will remember your choice for next time for repeatable daily, and weekly quests. Simply right-click the entry in the quest log list to access the 'always show' option.