This achievement requires that you open a cumulative total of 200 chests. Chests are scattered around the game world. They are mostly found near rock formations, buildings, or objects of some sort and are rarely found in the open. Opening a chest is done by clicking on it and solving the simple puzzle. After opening a chest, it remains open for some time, then respawns closed later on, allowing for the same chest to be opened an infinite number of times. Always be on the lookout for chests when your are traveling around the world and completing missions, especially in towns. Not only for this achievement, but they also give you a small amount of gold and a pack of ammo.