These quests are all over the map indicated by an orange marker with a little question mark. Once you reach said marker, you will speak with a member of the tribe, who will send you to another location indicated by the same orange marker (without the question mark) where you will start the quest.
These quests vary. Some are "Save Captive" quests, "Escort Wenja" quests or "Beast kill" quests.
Complete 15 to unlock the achievement.