This achievement requires you to complete 200 quests (new quests are indicated by an icon in the upper left of the screen with an !), which you'll do throughout the course of the game. Quests come in a couple of different forms, but they tend to include things like:
* Explore location(s) a certain number of times
* Explore location(s) at night
* Kill a certain number of the wandering creatures
* Find one or more items
* Complete a collection